Writing Chemical Formula

Chemical formula is a shorthand way of writing chemical compounds.  Instead of writing the full name of a compound chemical formula is written.  Instead of saying water, carbon dioxide, sodium chloride, just write the formulas:  H2O, CO2 and NaCl respectively.

In writing chemical formula, you must familiarize first with the different ions, the polyatomic ions and monoatomic ions.  Ions can be positive charged called cations and negatively charge called anions.  If  you want to have the list of these different polyatomic and monoatomic ions refer to my post Some Common Monoatomic and Polyatomic Ions.

Simple Rules in Writing Chemical Formula

A. Binary Compounds of metal and nonmetal

1. Determine the ions present in the compound, what is the cation and what is the anion.  Also determine the corresponding oxidation state of the elements.

Example:   What is the chemical formula of  Magnesium chloride?

In the above compound magnesium chloride, it is understood that the elements present are magnesium and chlorine, and it is expected that the first mention element is a metal and must be positive while chlorine is a nonmetal and the oxidation must be negative.

Based from the list of Monoatomic ions / Periodic Table, magnesium has positive charge

Watch video on writing chemical formula for binary compounds of metal and nonmetal HERE

2. Now in writing chemical formula we have to make sure that the charge is equal to zero or must have a neutral charge.  Since magnesium has a charge of +2 and chlorine is -1 we have to do something to make the charges equal.  The easiest way is to use the  criss cross method wherein the oxidation state of the positive ion will be the subscript of the negative ion and the oxidation state of the negative ion will be the subscript of the positive ion.  As shown below:

And to check the charges Mg will now have +2 charge while chlorine will have -2 charge and that makes the net charge equal to zero.  And therefore the final formula for magnesium chloride is 

B.  Binary compounds of two nonmetals

In writing the formula of binary compounds of  nonmetal just pay attention of the prefixes used.  Whatever is the prefix used is equal to the subscript of the element.  If no prefix involved it is understood to be 1 and there is no need to write the subscript.

Example:   What is the chemical formula of  dinitrogen pentoxide?

In the given name of the compound it is clear that nitrogen has a subscript of two and the oxygen has subscript of 5.  (To familiarize with the different prefixes refer to this post Simple Rules in Naming Chemical Compounds).

And so the formula of dinitrogen pentoxide is  

C.  Ternary Ionic Compounds

Ternary ionic compounds mostly composed of metal and polyatomic anion or polyatomic cation  and polyatomic anion.  Simply follow the rules in writing formula for metal and nonmetal but parenthesis is used when a polyatomic ion has subscript of 2 and above.

Example. What is the chemical formula of Copper (II) phosphate?

Simply determine the ions present in copper (II) phosphate compound.  In writing the ions always start with the positive ion followed by the negative ion.

Then use the criss cross process to determine the subscript of each ion.

And the final chemical formula is 

.

Take note of the presence of parenthesis, it is only use if the subscript of polyatomic ion is more than 1.  But if the subscript is only 1 no need to add parenthesis.

D.  Binary Acids

In binary acids, it is understood that hydrogen is paired with nonmetals except oxygen.  The formula starts always with hydrogen and followed by the nonmetal in the acid.

Example:  What is the formula of hydrosulfuric acid?

Based from the problem sulfur is the nonmetal that is paired with hydrogen.  Write the corresponding ions starting from hydrogen and follow the rule in writing chemical formula for metal and nonmetal compound. 

Following the criss cross process,

And the final formula is 

.

E.  Ternary Acids or Oxyacids

Ternary acids or oxyacids are acids where hydrogen ion is paired with polyatomic anion.  The formula always starts with H and followed by the needed polyatomic anion.   Once the name of acid ends with  -ic acid, that indicates that the polyatomic anion used ends with -ate, and if ends with -ous acid the polyatomic anion used ends with -ite.  (Just refer to this post Monoatomic and Polyatomic Ions for the different polyatomic anions).

Example:  What is the chemical formula of Nitric acid? 

Nitric acids is believed to have H ion and nitrate ion because of the ic ending.  

Using the criss cross process, the subscripts will be

And the final formula is

.

TRY THIS:

Write the formula of the following:

1.  Aluminum chloride

2.  Carbon tetrachloride

3.  Sodium phosphate

4.  Calcium carbonate

5.  hydrophosphoric acid

6.  hydroiodic acid

7.  Iron (III) sulfate

8.  Lithium oxide

9.  Phosphorous acid

10.  Chloric acid

FOR ANSWER'S KEY CLICK HERE

Laws of Gases: Boyle's Law

In studying the laws of gases there are different variables that we need to consider.  These are the volume, pressure, temperature, and the number of moles.  Volume is the amount of space occupied by a three-dimensional object as measured in cubic units (such as quarts and liter).  Pressure is the force applied in a given area, usually has the unit of atmosphere (atm), millimeter of  Mercury (mm Hg), torr, Pascal (Pa), kilopascal (kPa). Temperature is the degree of hotness and coldness measured on the definite scale such as Kelvin, ^oF and ^oC.  In solving problems related to gases, kelvin always is used.  Number of moles refer to the amount of substance, the unit is mol.

Boyle's Law

Robert Boyle is a British chemist who investigated the relationship between pressure and volume at constant temperature.  He used the J- shaped tube in studying the relationship between pressure and volume, he observed that adding pressure to the open end of the J-shaped tube at constant temperature will cause the decrease in the volume as shown in the figure below:

Boyle's law therefore can be stated as " if the volume of a fixed amount of gas is inversely proportional to volume at constant temperature." Inversely proportional mean that if you increase the pressure the volume will decrease or if the pressure is decreased the volume is increased.  If the relationship will be plotted in a graph, the graph will look like the figure below:

The mathematical equation that will express the inverse relationship between pressure and volume is

where ∝ is means proportional to.  We can change ∝ to an equal sign, 

where k₁ is the proportionality constant.   Arranging the equation:

This means that the product of pressure and volume at constants temperature is constant.

Using Boyle's Law we can predict the new pressure if there is a change in volume and at the same time volume can also be predicted if the pressure is also changed provided the temperature is not changed..  Since P₁V₁ = k, therefore P₂V₂ = k, combining both, the formula will be

P₁V₁ = P₂V₂

Using the above equation as the mother equation we can already formulate formulas for calculating P₁, V₁, P₂ and V₂.

These are the formulas:

What is atmospheric pressure?

Atmospheric pressure is the pressure exerted by earth's atmosphere.  The value of atmospheric pressure  depends on location, temperature and weather condition.  The atmosphere is much denser  near the surface of earth  than at high altitudes,  The denser the air is, the greater the pressure exerted.

Barometer is an instrument  used for measuring atmospheric pressure.  It was invented in 1643 by an Italian scientist named Evangelista Torricelli (1608-1647), who had been a student of the famous astronomer Galileo.  Below is the Torricelli's Barometer:

Standard atmospheric pressure, corresponds to the typical pressure  at sea level, is the pressure sufficient  to support a column of mercury 760 mm high at 0^oC at sea level.  In SI unit this pressure is equal to 101,325 Pa.

Standard atmospheric pressure defines some common non-SI units used to express gas pressure, such as atmosphere (atm), millimeter of mercury (mm Hg), and torr.

1 atm  =  760 mm Hg = 760 torr = 101,325 Pa = 101.325 kPa = 1.01325 bar = 14.69 psi (pounds per square inch,

Manometer is a device used to measure the pressure of gases other than the atmosphere.  The principle of operation is similar to that of barometer.  There are two types of manometer, the closed-tube and the open-tube manometer.

-Open-tube manometer is suited for measuring pressures equal to or greater than atmospheric pressure.

-Closed-tube manometer is used to measure pressures below atmospheric pressure.

Below are the models of the two types of manometer:

a.  Closed-tube manometer
b.  Open-tube manometer

Pressure Conversion

Sample Problem:

1.  Convert a.  0.075 torr to kPa

                   b.  6.6 x 10^-2 torr to atm

                   c.  749 mm Hg

Solution:

a.

b.

c.  

     

         

Molar Mass

Molar mass is the mass in grams of 1 mole of a substance .  It can be calculated by summing all the masses of all the atoms present in a compound.  For an element, the molar mass is equal to the atomic weight.

Mass of 1 mol of C  =  12.01 g
Mass of 1 mol of CH₄ = 16.04 g

The molar mass in grams per mole of any substance is numerically equal its formula weight in atomic mass unit.

1 mole of NaCl = 58 g/mol, molar mass
                         =  58 amu, formula weight

1 mole of CO₂ =  44 g/mol, molar mass
                         =  44 amu, fomula weight

1 mole of Al  =  27 g/mol, molar mass
                      =  27 amu, formula weight

Calculating Molar Mass

Sample Problem 1

What is the molar mass of glucose, C₆H₁₂O₆?

Solution:

Sample Problem 2
Calculate the molar mass of  Ca(NO₃)₂.

Sample Problem 3
Calculate the molar mass of CaCO₃

Sample Problem 4
Calculate the molar mass of SO₂.

Sample Problem 5

Calculate the molar mass of caffeine, C₈H₁₀N₄O₂.

Interconverting Masses and Moles

Since you know already how to calculate the molar mass of different substances, you are now ready to solve for mole and mass using molar mass of the compound.

You can use the formulas below:

No. of moles  =  mass ÷ molar mass

mass =  no. of moles x molar mass

or you can use the Factor Label Method of Calculating Problems.


Sample Problem 1
Calculate the number of moles of glucose (C₆H₁₂O₆) in 5.380 g of glucose.

Solution:
From the calculated molar mass of glucose above  is 180 g/mol.


Sample Problem 2
Calculate the mass in grams of 0.433 mol of calcium nitrate, Ca(NO₃)₂.

Solution:
The calculated molar mass of calcium nitrate is 164 g/mol.



Sample Problem 3

What is the mass in grams of a) 6.33 mol of NaHCO₃  b) 3.0 x 10^-5 mol of H₂SO₄.

Solution :
a.)  Calculating the molar mass of NaHCO₃

b.  Calculating the molar mass of  H₂SO₄