Valence Electrons

What is valence electrons?

Valence means last, therefore valence electrons are the electrons located at the highest energy level.  Valence electrons are the one responsible for chemical bonding and also responsible for the emission of light or energy.

How do we determine the valence electrons of the elements?

There are two ways we can determine the valence electrons of an element, one of these is with the use of electron configuration.   If you are using electron configuration, you need to determine the highest energy level and count the number of valence electrons found in the highest energy level.  But these is only true for those Representative elements or the group A elements.

Let us have an example

1.  H, atomic number is 1

     Electron Configuration:   1s¹

     Since hydrogen has only one electron, therefore the valence electron of hydrogen is only 1.

2.  He, atomic number is 2

     Electronic Configuration:  1s²
     Valence Electron :  2

    Since helium has only one sublevel and orbital, valence electrons is 2.

3.  F, atomic number = 9
     Electron Configuration:   1s² 2s² 2p⁵
     Valence electrons :   7

     The valence electrons of Flourine is 7 , the highest energy level is 2 having two sublevels the s and p.  Two electrons in 2s sublevel and 5 electrons in 2p sublevel, which equals 7 valence electrons.  We need to add since both are in the same energy level.

4.  Ca, atomic number = 20
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
     Valence electrons:  2

5.  Al, atomic number = 13
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p¹
      Valence electrons:    3

Relationship Between the Valence Electrons and the Group Number

Representative elements (Elements from group IA to group VIIIA now group 0) valence electrons are related to their group number.  Based from the examples above, Hydrogen being in group IA has 1 valence electron, Flourine in group VIIA has 7 valance electrons, Calcium in group IIA has 2 valence electrons and aluminum group IIIA has 3 electrons.  Except helium which is an exception. In other words, valence electron is just equal to the group number of elements in Representative group of elements but not related to transition elements.

Below are some examples of elements:

Electron Configuration

Electron configuration is the arrangement of electrons in successive sublevels and orbitals.   To determine the electron configuration of the elements electron distribution mnemonics is used.

In the electron distribution mnemonics the sublevels are already arranged according to increasing energy as shown below: 

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f  6d 7p

Now in writing electronic configuration we have to follow the Pauli's Exclusion Principles which states that, no two electrons can have four quantum numbers, and that orbitals can only accommodate 2 electrons with opposite spins.    Since, we only have 1 s orbital in s sublevel, only two electrons can occupy in s sublevel, and we have 3 p orbitals in p sublevel, and therefore total of 6 electrons can be accommodated in p sublevel.  In d sublevel there are 5 d orbitals, accommodating 2 electrons in each orbital there are 10 total electrons that can be accommodated in d sublevel, and in f sublevel there are 7 f orbitals and having  2 electrons each orbital, total of 14 electrons are in sublevel f.  An so we have to take note in writing electrons configuration the following:  s can have only 1 or two electrons, p can have only 1 to 6 electrons, d can have 1 to 10 electrons, and f sublevels can have 1 to 14 electrons.  

We use atomic number in writing the electron configuration, since atomic number is the number of protons at the same time the number of electrons.  Let us have an example:

What is the electron configuration of first 10 elements? 

₁H has only 1 electron and so the the electron is only located at the lowest sublevel 1s.  The electron configuration is :  1s¹

₂He has 2 electrons. The electron configuration is 1s²

₃Li :  1s² 2s¹

₄Be:  1s² 2s²

₅B : 1s² 2s² 2p¹

₆C : 1s² 2s² 2p²

₇N : 1s² 2s² 2p³

₈O : 1s² 2s² 2p⁴

₉F :  1s² 2s² 2p⁵

₁₀Ne : 1s² 2s² 2p⁶

We have to take note that in writing electron configuration, electron distribution mnemonics must be followed.

Energy Levels, Sublevels and Atomic orbitals

Based from the flame test, Neils Bohr compared the model of an atom with that of the solar system. The nucleus of an atom is comparable to the sun the center of the atom and the energy levels are comparable to the orbits and the electrons are comparable to the planets.   Energy levels of an atom is where the electrons occupy.  Based from the Periodic Table of elements the biggest atom has 7 energy levels.  This principal energy levels of the atom has corresponding energy based from Bohr Theory, and and also composed of different sublevels and with different kind of orbitals.  As the energy level increases the energy required also increases.

 Below is a table showing the different principal energy levels with their corresponding sublevels, orbitals and number of electrons:

As you will notice the number of sublevels are just the same as the number of energy levels, and that the name of the sublevels are also the same as the name of orbitals.  In determining the number of orbitals n² is used, where n represents the energy level and the maximum number of electrons can be determined using 2n², where n is also the number of energy levels.

The sublevels are 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 5s, 5p, 5d, 5f, and 5g.  The last sublevel the 5g is not yet existing in the modern periodic table  There are also sublevels in 6th and 7th energy levels which will be shown below. These sublevels are not yet arranged according to their energies, the electrons distribution mnemonics found in the periodic table arranged the different sublevels according to increasing energy.

The above electron distribution mnemonics can be used to write electron configuration and is already arranged according to increasing energy.  There is an overlapping in some sublevels, example 4s come first before 3d.  This is the arrangement following the above Mnemonics:

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

Flame Test

We know that atom is composed of three fundamental particles, the proton, electron and neutron.  And that the protons and neutrons are located inside the nucleus and electrons are found outside the nucleus.  Have you seen fireworks display? What are the colors produced? Yes different colors are produced in fireworks display.  What fundamental particle is responsible for the color of the fireworks display? Its the electrons and every element has its unique color of the flame.

Flame test is used to determine the elements present in a compound, because every element emits unique color of the flame which is used to identify the elements.  Below are some elements with their corresponding flame color.

boron -green
calcium - orange
sodium - yellow orange
potassium - light violet
copper - blue-green
barium - pale green
lithium - red
magnesium - white sparks
iron - yellow sparks
arsenic - blue
cesium - blue
copper (I) - green
strontium - crimson
thallium - pure green

How does the flame color produced?

How does the color of the flame produced?  What happens inside the metal atom?  As the metal atom is heated the electrons outside the nucleus are the ones that receive the  heat, electrons then jump to higher energy level upon absorption of heat.  As the electron jumps to higher energy level, the electron is said to be in the excited state, and in that state the electron becomes unstable, the tendency the electron will go back to lower energy levels emitting the excess energy in the form of light.  And this was the basis of Neil's Bohr Model of an atom.

Chemical Formula

Chemical formula shows the kind and number of atoms present in molecules or ionic compounds. Knowing the formula of a compound, you can determine the number of atoms and kinds of atoms present in a compound.  For example, the formula of water, H₂O, will give you an information that there are 2 atoms of hydrogen and only one atom of oxygen, meaning the ratio of the elements also is shown in chemical formula.

Chemical formula can be molecular formula and empirical formula.  Molecular formula shows the exact number of atoms of each element in a molecule.  Empirical formula shows the simplest ratio of elements in a compound or molecule.

Rules in Writing Chemical Formula 

Compounds can be binary compounds consisting only of 2 elements combined chemically, and ternary compounds composed of 3 or more elements chemically combined.   Example of binary compound is NaCl composed only of  sodium and chlorine; while CaCO₃ is an example of ternary compound composed of calcium, carbon and oxygen elements,

A.  Binary compounds
     In writing the chemical formula of binary compounds you have to follow the rules below:
1.  Identify the monoatomic cation and monoatomic anion that are present in a compound.  Example, magnesium chloride, this compound is composed of magnesium ion and chloride ion or chlorine ion. Start by writing the symbol of cation (positive ion) and followed by the symbol of anion (negative ion).

                Mg⁺²      Cl^-1

 2.  In writing chemical formula, the total charge must be equal to zero.   To balance the charges between the 2 ions above, the best way is to use the "criss cross process", which means that the oxidation number of the cation will be the subscript of the anion, and the oxidation number of  the anion will be the subscript of the cation, disregarding the sign of the numbers.

                  Mg₁⁺²     Cl₂^-1 

Analyzing the charges, magnesium will have +2 charge and the chlorine will have -2, with a total charge equal to 0.

Finalizing the formula, we have:
                  MgCl₂

B. Ternary Compounds
     In ternary compound, it has the same rules as that of binary compound, only that parenthesis is used for polyatomic ions having 2 or more subscript.
1. Identify the monoatomic or polyatomic cations so as the anions. Also write the symbol or fomula of the cation followed by the anion.  For example the compound Iron (III) sulfate, it is composed of iron having +3 oxidation number and sulfate ion (SO₄^-2).
          Fe⁺³  SO₄^-2

2.  The same as that of the binary compound, the charge of the chemical formula of ternary compound must be equal to zero. So, to balance the charges the crisscross process also will be used.   The oxidation of iron which is 3 will be the subscript of sulfate and the oxidation of sulfate  will be the subscript of iron.

          Fe₂⁺³  SO_{4 3}^-2

Analyzing the charges, Fe will have a charge of +6 and sulfate will have -6, with a total charge of 0.

In writing chemical formula, polyatomic ion, although composed of 2 or more elements, just act as one in chemical formula writing, therefore the subscript 3 must not only belong to oxygen but also for sulfur, to do so parenthesis is used, as shown below:

          Fe₂(SO₄)₃

If the subscript of the polyatomic ion is equal to one, it is not written because absence of subscript means one.  On the other hand, parenthesis also should not be written if the subscript is equal to 1.

Since writing chemical formulas involve balancing the charges between the cation and anion, once the charge is already equal, no subscript is added to both cation and anion.  Like for example the compound calcium carbonate, composed of calcium ion and carbonate ion.

         Ca⁺²   CO₃^-2

Since both have the same oxidation number, meaning the charge is already equal, the formula will be

         CaCO₃

Circulatory System

Circulatory system is the life support structure that delivers oxygen and nutrients to the cells of the different parts of the body and also transport waste away from the body.  It is composed of three major parts the heart, blood, and blood vessels.

The three major parts of the circulatory system and their functions:

Heart - pumps blood throughout the body.
Blood - carries oxygen, and nutrients to the different parts of the body.
Blood vessels - carries the blood throughout the body.  Blood vessels are classified as:

• Arteries -carry oxygenated blood away from the heart to the cells, tissues and organs of the body.
• Veins - carry deoxygenated blood back to the heart.
• Capillaries - are very tiny blood vessels that form a connection between arteries and veins.  This is where the exchange of gases occur.

Types of Circulation

1. Pulmonary circulation - is the movement of the blood from the heart to the lungs and back to the heart. 
2. Coronary circulation - is the movement of the blood through the tissues of the heart.
3. Systemic circulation - is the movement of the blood from the heart to the different parts of the body excluding the lungs.

The Human Heart

The heart is a muscle that is the same size of your fist.  It has four chambers, each chamber has corresponding function.  These are the right atrium, left atrium, right ventricle and left ventricle.

The right atrium receives the deoxygenated blood from the different parts of the body; the left atrium, receives the blood from the lungs; the right ventricle, pumps the blood to the lungs; the left ventricle, pumps the blood to the different parts of the body.  

There is a valve between atria and ventricles, to prevent the blood from flowing backwards.  The valves are like one-way doors that keep the blood from moving in one direction.  Valves control movement of blood into the heart chambers and out into aorta and pulmonary artery.

Valves of the Heart

• Tricuspid valve - is located between right atrium and right ventricle
• Pulmonary valve- is located between the right ventricle and pulmonary artery
• Mitral valve - is located between the left atrium and left ventricle
• Aortic valve is located between the left ventricle and the aorta

How the heart works

The right atrium receives the blood from the different parts of the body, and will pass to the right ventricle in which is the once responsible of pumping the blood to the lungs.  From the lungs the blood will again go back to the heart entering in the left atrium and to the left ventricle, and then it will be pump to the different parts of the body, and will then again go back to the heart for another cycle.  

Respiratory System

Respiratory system is composed of different organs in our body that are responsible for breathing. Breathing is a process of taking in oxygen and expelling carbon dioxide from our body.  The parts of the respiratory system that are responsible of supplying oxygen are nose, nasal passages, trachea, bronchi, lungs and diaphragm.

Different Parts of Respiratory System and their  Function

Nose and nasal cavity  is the organ through which the air enters and filtered.  The hair that line the inside wall are part of the air  cleansing system.

Pharynx collects incoming air from the nose and passes it downward to the trachea (windpipe).

Epiglottis is a flap of tissue that guards the entrance to the trachea.  It closes when anything is swallowed that should go into the esophagus and stomach.

Larynx (voice box) contains the vocal cords. When moving air is breathed in and out, it creates voice sounds.

Trachea (windpipe) is the passage way of air into the lungs.

Bronchi are two branching tubes that connects the trachea to the lungs.  The bronchial tubes are line with cilia (like very small hairs) that have a wave-like motion.  The motion carries mucus (sticky phlegm or liquid) upward and out into the throat, where it is either coughed or swallowed.  The mucus catches and holds much of the dust, germs and other unwanted matters that harms the lungs.  The lungs get rid of the mucus through coughing.

Bronchioles is the finer subdivision of bronchi, hairlike tubes that connect to the alveoli. 

Alveoli are also called air sacs, it is where the gas exchange occurs.  Each of the air sacs is covered by tiny blood vessels called capillaries.The capillaries are connected to a network of arteries and veins that move blood through your body.

Diaphragm is a strong wall of muscles that separates the chest cavity from the abdominal cavity. It is the main muscles used for breathing.  The diaphragm expand and contract the thoracic cavity, causing the lungs to expand and contract.

THE RESPIRATORY SYSTEM

How Does Respiratory System Works

The air enters through the nose, where it is filtered and to the nasal passages where gas is warmed and moistened.  Then to the windpipe or trachea and down to the bronchi and to the bronchioles and to the alveoli.  In the alveoli the oxygen is diffused through the capillaries and to the heart.  On the other hand, carbon dioxide is also diffused from the capillaries in the alveoli to the bronchioles and out of the lungs to the bronchi, trachea and passes out from the nose. 

Colligative Properties of Solution

Colligative properties of solution are properties of solution that depend on the amount of solute present in the solution and not on the nature of solute particles.  These are the vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure.

Vapor-pressure Lowering

Some liquids like water contains molecules,  these molecules when absorb enough energy will eventually form into vapor.  The pressure exerted by the vapor is called vapor pressure.  Try boiling water in whistling tea kettle, you will notice that when it boils the tea kettle whistle, this is because of the pressure caused by the vapor.   What will happen to the vapor pressure when solute is added to the solvent?  Since solution contains already solute that is attached to the solvent, the vapor pressure of the solution is lower compared to the pure solvent.  The relationship is expressed  by Rault's Law, which states that the partial pressure  of a solution, P_A, equals the product of the mole fraction  of the solvent in the solution, X_A, times the vapor pressure of the pure solvent, P^o_A.

P_A  =  X_A P^o_A

For example, the vapor pressure of pure water at 20^oC is 17.5 torr.  What if glucose (C6H12O6) is added to water.  Let us say the mole fraction of water, X_H2O = 0.800 and mole fraction of glucose, X_C6H12O6 = 0.200.  What is the vapor pressure of  the solution?  Calculating the problem:

P_H2O = X_H2O P^o_H2O

           = (0.800)(17.5 torr) 

           = 14.0 torr

Based from the calculation above, there is a decrease in vapor pressure from 17.5 torr of pure solvent to 14 torr of vapor pressure of the solution.

Sample Problem:
Calculate the vapor pressure of a solution made by dissolving 218 g of glucose (molar mass = 180 g/mol) in 460 mL of water at 30^oC.  What is the vapor pressure lowering?  The vapor pressure of water at 30^oC is 31.82 mm Hg.  Assume the density of the solution is 1.00 g/mL.

Solution:

First, calculate the number of moles of glucose and water.

Then,  the mole fraction of water, X_1(water) is

For the vapor pressure of the solution, we have,

P_A  =  X_A P^o_A
Psolution = (0.955) (31.82 mm Hg)
                = 30.4 mm Hg


Therefore the vapor pressure lowering is 31.82 mm Hg - 30.4 mm Hg =  1.4 mm Hg

Boiling Point Elevation

Boiling point  is the temperature at which the vapor pressure equalizes with the atmospheric pressure.  Since the vapor pressure of the solution is lowered than its pure solvent, therefore the boiling point also of the solution is affected with the presence of the solute.  Below is the phase diagram illustrating the boiling point elevation and freezing point depression of aqueous solution:

Based from the graph above, the boiling point of solution is higher than that of the boiling point of pure water. The increase in boiling point is due to the solute present in the solution.  The normal boiling point of pure liquid  is the temperature  at which its vapor pressure equals 1 atm.  Since vapor pressure of the solution is lowered , it requires  higher temperature to attain vapor pressure of 1 atm. Thus, the boiling of the solution is higher than that of the pure liquid.  

The increase in boiling point relative to that of the pure solvent, is directly proportional to the number of solute particles per mole of solvent molecules, meaning directly proportional to the concentration expressed in molality.  Thus,

∆T_b = K_bm

where ∆T_b is boiling point elevation, m is molality of the solution, and K_b is the molal boiling-point elevation constant.  The unit of K_b is ^oC/m.

Below is a table showing the Molal Boiling-Point Elevation and Freezing-Point Depression Constants: 

Freezing-Point Depression

Freezing-point of a solution is the temperature at which  the first crystals of pure solvent begin to form equilibrium with the solution.  Based from the graph above, the freezing point of the solution is lower than that of the pure liquid.  This is because freezing involves a transition from disordered to ordered state wherein, energy must be removed from the system in order the process to occur.  Because solution has greater disorder than the solvent, more energy is needed to be removed from it in order to create order the same as that of the pure solvent. 

The freezing point depression, ∆T_f,  is directly proportional to the molality of the solute:

∆T_f = K_f m

where,  m is the concentration of the solute in molality, K_f is the molal freezing-point depression constant.  The unit also is ^oC/m.  

Sample Problem:

Automotive antifreeze consists of ethylene glycol (C₂H₆O₂), a nonvolatile nonelectrolyte.  Calculate the boiling point and freezing point of a 25.0 mass % solution of ethylene glycol in water.

Solution:  

The given in the problem is 25 % by mass of ethylene glycol solution.  Let us assume an amount of solution let say 1000 g of the solution.  Since the solution is 25 % by mass, this means that 250 g of ethylene glycol is in 750 g of water.  Using this quantities we can calculate the concentration of the solution in molality:

After calculating the molality, boiling point elevation and freezing point depression can now be calculated:

∆T_b = K_bm = (0.51 ^oC/m)(5.37 m) = 2.7 ^oC

∆T_f = K_f m = (1.86 ^oC/m)(5.37 m) = 10.0 ^oC

boiling point = (normal boiling point of solvent) + ∆T_b     
                      =  100 ^oC +  2.7 ^oC =  102.7 ^oC

Freezing point = (normal freezing point of solvent + ∆T_b
                        =  0.0 ^oC  - 10.0 ^oC =  -10.0 ^oC

Osmotic Pressure

Osmosis is the net movement of solvent molecules through semipermeable membrane from dilute solution to a concentrated solution.  Semipermeable membrane is a membrane that allows the passage of solvent molecules but block the passage of solute molecules.  Let us look at the figure below:

The figure above,  a is a container containing the same amount of pure solvent and solution  separated by a semi permeable membrane.  The left part contains the pure solvent while the right part contains the solution.  In container b, osmosis occurs, the solution part rises, this is because the solvent in the left compartment passes through the semipermeable membrane .  Osmotic pressure is equal to the hydrostatic pressure exerted by the column of fluid in the right tube at equilibrium.           

Osmotic pressure 𝝅 of the solution is the pressure required to stop osmosis.

The osmotic pressure of a solution is given by:

𝝅 = MRT  

where M is the molarity of the solution, R is the gas constant (0.0821 L.atm/K.mol) and T is the absolute temperature.  The osmotic pressure, 𝝅, is expressed in atmospheres.

Osmotic pressure, the same with other colligative properties, is directly proportional to the concentration of solution.  This means that all colligative properties of solution are dependent on the number of particles of solute present in the solution.  If two solutions have the same concentration and osmotic pressure, they are said to be isotonic.  If two solutions have different osmotic pressure, the more concentrated solution is said to be hypertonic, and the more dilute solution is said to be hypotonic.  

Osmosis plays a vital role in living systems.  Example, the membrane of red blood cells are semipermeable.  When red blood cells is placed in a solution that is hypertonic  relative to the intracellular solution (the solution within the cells) causes water to move out of the cell.  This causes the cell to shrivel, a process called crenation.  Placing the cell in a solution that is hypotonic relative to the intracellular fluid causes water to move into the cell.  This causes the cell to rupture, a process called hemolysis.  Some patients who need body fluids or nutrients replaced but cannot be done orally  are given solutions by intravenous (IV) infusion, which feeds nutrients directly into the veins.  To prevent crenation or hemolysis of red blood cells , the IV solutions must be isotonic with the intracellular fluids of the cells.

There are many interesting examples of osmosis, one of these is watering of plants.  Water moves from soil into plant roots and into the upper portions of the plants.   But if you will use seawater in watering the plants, the reverse will occur.  The water from the plant will flow down into the roots into the soil, this will cause the dehydration of plant.  This is because in osmosis the water moves from less concentrated to the more concentrated solution.

Sample Problem:

The average osmotic pressure of blood is 7.7 atm at 25^oC.  What concentration of glucose (C₆H₁₂O₆) will be isotonic with blood?

Solution:
The given above are the osmotic pressure and the temperature.  Temperature must be converted to K before calculating.

      𝝅 = MRT  

Concentration of Solution

Concentration is defined as the amount of solute present in a given amount of solvent.  It can be expressed qualitatively and quantitatively.  The qualitative way of expressing the concentration is either diluted or concentrated.   A concentrated solution contains more solute in the solution while diluted solution contains less solute in the solution.  Very sweet sugar solution is an example of concentrated solution while sugar solution with little sweetness is an example of diluted solution.

There are many different ways in which we can express concentration quantitatively; some are these are the following: percent by mass, percent by volume, molarity, and molality.


Percent by Mass

Percent by mass (% by mass) is defined as the mass of the solute per mass of solution multiplied by 100.  In formula it can be written as

The formula above can be used when the given in the problem are the mass of the solute and the mass of the solution.   But if the given in the problem are the mass of solute and mass of solvent, you can use this formula:

Sample Problem 1.

A solution is made by dissolving of 13.5 g of glucose in 100 g of water.  What is the mass percentage of solute in this solution?

Solution:

Given:

13.5 g of glucose, mass of solute

100 g of water, mass of the solvent

Formula and calculation:

Sample Problem 2.

What is the percent by mass of the solution when 5.50 g of NaBr is dissolved in 78.2 g of solution?

Solution:

Given:

5.50 g of NaBr, mass of solute

78.2 g of solution, mass of solution

Formula and calculation:

Percent by Volume

Percent by volume is just the same with that of percent by mass, only that volume of the substance is being used. Percent by volume can also be calculated using the formula below:

   

Sample Problem 1.

What is the percent by volume of the solution when it contains 27 mL of alcohol in 100 mL of solution?

Solution:  

Given: 

27 mL alcohol, volume of solute

100 mL solution, volume of solution

Formula and calculation:

Degree Proof

Degree Proof is another measure of concentration related to percent by volume.  It is twice the percent by volume.  

This means that an 80 degree proof liquor contains 40 % of alcohol by volume.  The table below shows the alcohol content of some alcoholic drinks:

Molarity

Molarity (M) is also known as molar concentration, it refers to the number of moles of solute per liter of solution.

The unit of molarity is mole per liter (mol/L) or simply M.

Sample Problem 1.

What is the molarity of an 85 mL ethanol (C₂H₅OH) solution containing 1.77 g of ethanol?

Solution:

Given:

1.77 g of ethanol

85 mL of solution = 0.085 L solution

To calculate molarity the number of moles of solute must be calculated, and to calculate the number of moles the molar mass of ethanol should also be calculated first.  

Molar mass of ethanol

C = 2 x 12 =  24 g 

H = 6 x 1   =   6  g

O = 1 x 16 =  16 g

                      46 g/mol

Number of moles can be calculated by dividing the given mass to its  molar mass.

To calculate the molarity 

Molality

Molality (m) is also called molal concentration is the number of moles of solute per kilogram of solvent.  The unit of molality is mol/kg or m.

Sample Problem:

A solution is made by dissolving 4.35 g of glucose (C₆H₁₂O₆) in 25.0 kg of water.  Calculate the molality of glucose in the solution.  

Solution:

Given:

4.35 g glucose

25.0 g of water = 0.025 kg water

We need to calculate the molar mass of glucose (C₆H₁₂O₆), before we can calculate the number of moles.  

C = 6 x 12 = 72 g

H =12 x 1 =  12 g

O = 6 x 16 = 96 g

                    180 g

moles of solute can also be calculated using  factor label method as shown below:

   

For more Sample Problems click HERE

Factors Affecting the Rate of Dissolution

Dissolution is  a process whereby solute is attracted to the solvent forming a solution.  Dissolution can either proceed faster or slower depending on the different situations.   What do you think are the factors affecting the rate of dissolution?

Temperature

Temperature affects the rate of dissolution most especially for solid in liquid solutions.  Try dissolving 1 tablespoon of sugar in two different glasses.  One glass containing hot water and the other glass with cold water.  Which glass will dissolve sugar faster?  Of course glass containing hot water.    But why?  The molecules in hot water have more kinetic energy than in cold water, therefore the interaction between solute and solvent is more faster than in the cold one and so the faster is the rate of dissolution.  Therefore, dissolution increases at a higher temperature, and decreases at lower temperature.


Particle Size or Surface Area

Does the size of the particles affects the rate of dissolution?  Which will dissolve more faster in a glass of water,  1 tablespoon of  finely fine iodized salt or the rock salt? Why?  The answer is the iodized salt because of it smaller particles size therefore, it has large exposed surface area that can immediately allow the molecules to interact faster with the solvent.  This means that the smaller the particle size or the larger the surface area the faster is the rate of dissolution.


Rate of Stirring

Stirring does also affect the dissolution process.  Stirring more rapidly the mixture, the faster is the rate of dissolution and the slow you stir the mixture, the slower is the rate of dissolution.   Why?  Stirring more rapidly the mixture also allows the interaction of solute and solvent faster.

Factors Affecting Solubility

Solubility is defined as the maximum amount of solute that will dissolve in a given amount of solvent at a specific temperature.  A solution which contains the maximum amount of solute is said to be saturated.  Solutes have different solubility in water and are affected by some factors.  For example, the solubility of NaCl in water at 0^oc is 35.7 g per 100 mL   This means that at 0^oC only 35.7 g of NaCl can dissolve in 100 mL of water.  Adding more solute to the solution will not anymore dissolve the solute.  Below is a graph showing the solubility of some salts:

Temperature

Temperature is one of the factors that affects the solubility of solute in a given solvent.  The solubility of solid in liquid is different with that of gas in liquid solution, which we need to emphasize.  For solid in liquid solution like salt in water, generally the solubility increases as the temperature increases,   except for some exceptions.  For example in the solubility versus temperature graph shown above, almost all salts increases solubility as the temperature increases, only Ce₂(SO₄)₃ decreases its solubility.

For gas in liquid solution, the increase in temperature results in decrease in solubility.  Example, carbonated drinks contains carbon dioxide dissolved in water.  It is usually placed inside the refrigerator to increase its solubility.  Try boiling carbonated drinks and compare its taste to the original mixture, and you will find a big difference in their taste, once heated the carbon dioxide dissolve in water comes out from the water and result in decrease in solubility

Pressure

Pressure does not affect all types of solutions, only in gas in liquid solution, like carbonated drinks.
Try opening a bottle of soft drinks and observe what happens.  An effervescence  indicates that there is  gas that comes out from the solution.  Opening the bottle means you decrease the pressure and the solubility also decreases.  The quantitative relationship between pressure and solubility is given by Henry's Law, which states that the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution.

c  ∝  P

c  =  kP

where c is the molar concentration in mole per liter of the dissolved gas;  P is the pressure in atmosphere of the gas over the solution; and for a given gas, k is a constant that depends on temperature.  The constant k has the unit mol/liter . atm.  

Nature of Solute and Solvent

The solubility of some solids and liquids follow the general rule like dissolves like.  Solutes and solvent can be polar, nonpolar or ionic.  Like dissolves like means polar solvent dissolves polar solutes and ionic solutes and nonpolar solvent dissolves nonpolar solutes.  The stronger the attraction between solute and solvent the higher the solubility.  For example, kerosene is immiscible in water, simply because they have different polarity, whereas if we will combine kerosene and gasoline, there is a complete dissolution that occurs because of similar nature of the two substances.  

Solution

Mixtures are around us.  We encountered them everyday.  Mixtures are either homogeneous and heterogeneous.  The homogeneous mixture is what we called solution.  Some examples of solutions are seawater, sugar in water, alcohol solution, and more.  Solutions maybe solid, liquid or gas.

Solution is a homogeneous mixture composed of solute and solvent.  Solute is a component present in smaller quantity while the solvent is a component present in greater quantity.  Solute and solvent can vary their quantities.  Example, alcohol solution can be 30% by volume and 70% by volume. A 30% alcohol solution means that in every 100 mL of the alcohol solution, contains 30 mL of alcohol and 70 mL water, while a 70% alcohol solution means there is 70 mL alcohol and 30 mL water.

Types of Solution

Solutions are classified into three types: the solid solution, liquid solution and the gaseous solution.  The type of solution can be identified based on their solvent.  If the solvent is liquid the solution is liquid, if the solvent is solid the solution is solid and if the solvent is gas the solution is a gaseous solution.  Below are some examples of solutions and their types.

Solution can also be characterized by their capacity to dissolve a solute.  We have saturated solution, a solution in which contains maximum amount of solute in a given solvent, at a specific temperature. Unsaturated solution contains less solute than it has the capacity to dissolve in a given solvent, at a specific temperature. Supersaturated solution contains more than the maximum amount of solute that it can dissolve in a given amount of solvent, at a specific temperature.  Supersaturated solution is not very stable.  In time, some of the solute will come out from the solution as crystals.  The process is called crystallization.  Crystallization is a process in which dissolved solute comes out of solution and forms crystals.

The Solution Process

Solution is formed when the solute disperses uniformly in the solvent.  That is when the attractive forces between solute and solvent particles are comparable in magnitude with those that exist between the solute particles themselves or between the solvent particles themselves.  For example, when NaCl dissolves readily in water, the attractive interaction between the Na⁺ ions and Cl^- ions and the polar H₂O molecules overcome the lattice energy of NaCl.

Below shows a molecular view of solution process taking place in three steps.  Step 1 is the separation of solvent molecules; step 2 also shows the separation of solute; and the step 3, is the mixture of solute and solvent.

Below shows what happens when NaCl is dissolved in water:

The picture shows that when NaCl is dissolved in water, the water molecules orient themselves on the surface of NaCl crystals.  The positive end of the water dipole is oriented toward the Cl- ions, and the negative end of the water dipole is oriented towards the Na+ ions.  The ion-dipole attraction between the ions and the water molecules are sufficiently strong to pull the ions in NaCl crystals. 

Periodic Table of Elements

Between the period of 1800 and 1900, there were more than half of the elements known today were already discovered.  And there were elements that are found to have similar physical and chemical properties, and these elements were grouped by some scientist according to a certain basis. The arrangement of elements according to similar physical  and chemical properties is called the Periodic Table of Elements.   

The Modern Periodic Table is already arranged according to increasing atomic number in which elements having similar physical chemical properties were grouped as one group.  Examples of these elements are lithium (Li), sodium (Na), potassium (K) are all soft and very reactive metals.  Helium (He), Neon (Ne), and Argon (Ar) are very nonreactive gases.  The Modern Periodic Table is shown below:

The Modern Periodic Table is divided into several vertical columns called group or families and horizontal rows called the periods.  Group or families are categorized as the A families (Representative elements) and the B families ( Transition elements).  The Group A elements are divided into 8 groups (group IA to group 0, previously known as the group VIIIA).  These groups have special name:  Group IA elements are called Alkali Metals, group IIA elements are Alkaline Earth Metals, group IIIA elements are called Boron Group, group IVA elements are called Carbon group, group VA elements are called Nitrogen group, group VIA elements are called the Chalcogen group,  group VIIA are called Halogen group and the group 0 elements (VIIIA) are called Noble gases.

The elements are also divided into three categories: the metals, nonmetals and metalloids or semi-metals.  Metals are found on the left corner of the periodic table.  They are known to be conductors of heat and electricity, malleable, ductile, luster, and have high melting and boiling point.  Nonmetals are located on the upper right corner of the periodic table.  They are known to be nonconductors of heat and electricity, brittle, low melting and boiling point, no luster.  Metalloids or semi-metals are elements that possess the characteristics of both metal and nonmetal, located in between metals and nonmetals.  You can locate the semi-metal elements on the periodic table presented above those elements with light blue in color.

Atomic Number, Mass Number and Isotopes

All atoms contain the three fundamental particles of matter, the proton, electron, and neutron.  But how do the atoms differ from one another?  Each element has its unique number of protons that serve as the identity of the element.  We can identify the number of protons of an element by knowing the atomic number of that element.  Atomic number (Z) is the number of protons in the nucleus of each atom of an element.  In a neutral atom the number of protons is equal to the number of electrons, therefore atomic number gives the number of protons and the number of electrons of an atom.  Example, Oxygen has an atomic number of 8, this means oxygen atom has 8 protons and 8 electrons.

Mass Number (A) is the total number of protons and electrons in the nucleus of an atom.  All atomic nuclei contain both protons and neutron.
 
                            Mass number  =   number of protons  +  number of neutrons

We can determine the number of neutrons if we know the atomic number and mass number of and element. The number of neutrons can be determine by subtracting the atomic number to the mass number. Example the element nitrogen, it has atomic number of 7 and mass number of 14.  Therefore, there are 7 protons and 7 neutrons.   How do we arrive at that answer?  Since the atomic number is 7 the number of protons is also seven.  To get the number of neutrons simply subtract 7 to 14 to get 7 (14 - 7 = 7).

In writing the atomic notation the atomic number is written on the lower left portion of the chemical symbol and the mass number on the upper left corner of the chemical formula.  As shown below:

Example of atomic notation of element are the isotopes of Uranium as shown below: 

As you can see above:  the element Uranium has different mass number.  Is that possible?  The atomic notation above is an example of  isotopes of uranium.  What is an isotope?  Isotopes are atoms of the same element having the same atomic number but different mass number.  It means that the number of protons are the same but different number of neutrons.  Other example of isotopes are the isotopes of hydrogen: the protium, deuterium, and the tritium. 

Sample Problem:
Determine the number of protons, electrons and neutrons of the following:
1.  Oxygen, Z= 8, A=17
2.  Hg, Z=80, A=199
3.  Hg, Z=80, A=200
4.  Cu, Z=29, A=63
5.  Fe, Z=26, A=56

Answer:
1. Oxygen has 8 protons and 9 neutrons
2.  Mercury-199 has 80 protons and 119 neutrons
3.  Mercury-200 has 80 protons and 120 neutrons
4.  Copper has 29 protons and 34 neutrons
5.  Iron has 26 protons and 30 netrons