THE MOLE & Chemical Formulas:
A Review

Okay, kids. If you don't remember this one, you can just give up and turn in your TI-83s right now.

A mole (also known as Avogadro's Number) is the number that is used in making calculations involving atoms and molecules. Given the relatively microscopic size of the particles, it would be rather difficult (read: utterly pointless) to calculate amounts in terms of numbers of atoms. So, we use the mole.

A mole, (mol for short) is equal to (all together now) 6.022 x 1023 atoms or molecules.

The atomic mass given for an element on the periodic table measured out in grams is equal to one mole of atoms of that element.

Thus, the Molar Mass (MM) of elements and compounds is the mass, in grams, equal to the atomic and formula masses of those elements and compounds. The unit of Molar Mass is grams/mole. Click here for Practice in Calculating Molar Masses.

Empirical and Molecular Formulas

Believe it or not, there is more than one kind of chemical formula (insert a sharp gasp of surprise here). Judging from the heading of this section, they would be empiricaland molecular formulas. Empirical formulas give the lowest whole number ratio of the atoms in a compound, whereas the molecular formula gives the exact composition of one molecule.

Empirical Molecular
HO H2O2
CH C2H2
CH C6H6
NO NO
NO2 N2O4

Empirical formulas can be calculated using experimental data:

• Given that a certain compound is 69.58% Ba, 6.090% C and 24.32% O, calculate the empirical formula of this compound.
• Assume that you have 100.00 g of the compound:
69.58% Ba = 69.58 g Ba
6.090% C = 6.090 g C
24.32% O = 24.32 g O
• Convert the mass of each element to moles of that element:
Ba = (69.58 g Ba)(1 mol Ba/137.3 g Ba) = 0.5068 mol Ba
C = (6.090 g C)(1 mol C/12.01 g C) = 0.5071 mol C
O = (24.32 g O)(1 mol O/16.00 g O) = 1.520 mol O

Divide through each value by the smallest number of moles to get a 1 : 1.001 : 2.999 ratio, which rounds of nicely to give the formula BaCO3

Molecular formulas can be calculated using empirical formulas:

• Given that the empirical formula of a compound is CH and the molar mass is 104 g/mol, calculate the molecular formula.
mass of C = 12.0 g/mol
mas of H = 1.01 g/mol
empirical formula mass = 13.0 g/mol

to find the number of CH units in the compound:

# CH units = (104 g/mol)(1 mol/13.0 g) = 8.00

Molecular formula = 8(CH) or C8H8

Send questions, comments or suggestions to
Gwen Sibert at the Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us