Mole Concept

Mole, which is symbolized n, and has a unit mol is the counting unit for number of atoms, ions or molecules.  One mole is the amount of substance (either in atoms, ions or molecules) as the number of atoms in 12 g of Carbon-12.  Scientists found out that 1 mole of any substance contains 6.02 x 10²³ particles. This number is called Avogadro's Number (N), in honor of the Italian Scientist Amedeo Avogadro (1776-1856) with the unit mol^-1.  Which means there are 6.02 x 10²³ particles per mole of a substance.

Example:

1 mole of C atoms = 6.02 x 10²³ atoms
1 mole of CO₂ molecules =  6.02 x 10²³ molecules
1 mole of SO₃^-2 ions  =  6.02 x 10²³ ions

Mole is just the same as dozen, 1 dozen of eggs = 12 pcs of eggs, 1 dozen of chairs = 12 pcs of chairs.  Anything that is 1 dozen always contains 12 pcs.  One mole on the other hand,  always contains 6.02 x 10²³ particles.

Interconverting  Moles to the number of Particles

Using the Avogadro's number we can determine the number of atoms, ions or molecules of a given substance.

1.  Calculate the number of C atoms in 0.350 mol of C₆H₁₂O₆.  

Solution:

2.  How many moles of CO₂ are in 3.1 x 10²⁴ CO₂ molecules?

Solution:

3.  Aluminum is a metal with a high strength-to-mass ratio  and a high resistance to corrosion.  Calculate the number of atoms of 0.371 mol of Al.

Solution:

4.  Cobalt (Co) is a metal that is added to street to improve its resistance to corrosion.  Calculate the number of moles of 5.00 x 10²⁰ atoms of Co.

Solution:

5.  How many oxygen atoms are in a) 0.25 mol Ca(NO₃)₂, and  b) 1.50 mol of Na₂CO₃?

a.

b.  

Intermolecular Forces of Attraction

Intermolecular forces are attractive forces between molecules.  This forces are responsible  for the non-ideal behavior of gases at high pressures and low temperatures.  This forces are also important in establishing  the form and behavior of matter.

What is the difference between intermolecular forces and intramolecular forces?  Intermolecular forces are forces that holds molecules together while intramolecular forces are hold atoms together in a molecule.  Intramolecular forces are the chemical bond, the covalent, ionic, and metallic.  When intermolecular forces is broken only change in phase occur but when intramolecular forces are broken chemical change occur.  With regards to the strength of these forces, generally intermolecular forces are weaker than intramolecular forces.

The different types of intermolecular forces are hydrogen bond, dipole-dipole, and London  dispersion forces (Dipole-dipole and London dispersion forces are also known as Van der Waals Forces).  This is after Joannes Van der Waals, who developed the equation for predicting the deviation of gases from ideal behavior.  All intermolecular forces are electrostatic which involve the attraction of positive and negative species like ionic bond but weaker than the ionic bond.

Hydogen Bond

Hydrogen bond is a special kind of dipole-dipole interaction.  This exist when hydrogen is attracted to highly electronegative atoms like N, O, F.  Example of this are the polar molecules of NH₃, H₂O and HF.

Above photo shows the hydrogen bond exist in water molecules.  This bond is quite stronger than dipole-dipole interaction due to some reasons.  One is that hydrogen is covalently bonded to high electronegative atoms which lead to special polar X - H bond.  Second is due to small size of hydrogen atom, the dipoles come close together and produce strong dipole-dipole interactions.  

Hydrogen bond are possible only with hydrogen containing compounds because all atoms other than H have inner-shell electrons to shield their nuclei from attraction by lone-pair electrons of the nearby atoms.  Only F, N and O only meet the hydrogen bonding formation due to their high electronegativity value.

Dipole -dipole interaction

Dipole-dipole forces are attractive forces that exist between polar molecules.  Polar molecules are molecules with dipole moments.  The larger the dipole moment the greater the force.  Below shows what happens during dipole-dipole interaction between molecules.  Molecules are arranged in a way that their positive pole is attracted to the negative pole of the other molecule.  

Example of molecules with dipole-dipole interactions are CO, HCl, PCl₃, HBr.

Dispersion forces

Dispersion forces are forces that exist between nonpolar molecules.  Examples of nonpolar molecules are the diatomic molecules, CO₂, CH₄.  This was proposed by Fritz London, a German-American physicist in 1930.  London recognized that the motion of electrons in an atom or molecule can create an instantaneous, or momentary, dipole moment. The strength of dispersion forces tends to increase with increasing atomic or molecular size. Below is an example of dispersion forces between Cl₂ molecules.