3day8 – Molecular Formula
The molar mass of a molecular compound is always a whole number multiple, x, of
the molar mass of the empirical formula.
You can determine the molecular formula by
1. Find x by dividing the molecular mass by the empirical mass
2. Multiply the subscripts in the empirical formula by this number to get the
molecular formula
Example: A white powder is found to have an empirical formula of P2O5. The
compound has a molar mass of 283.88g. What is the compound’s molecular
formula?
1. Find molar mass of the empirical formula
P= 2(30.97) = 61.94
O = 5(16.00) = 80.00
MM =
141.94
2. Divide MM by empirical MM
283.88/141.94 = 2
3. Multiply subscripts
P2O5  P4O10
Example: A compound has an experimental molar mass of 78.12 g/mol. Its
empirical formula is CH. What is its molecular formula?
Molar mass of the empirical formula
C = 1(12.01) = 12.01
H = 1(1.01) = 1.01
MM = 13.02
Find multiple:
x = 78.12/13.02 = 6
Multiply subscripts:
CH  C6H6
Class work: Do the worksheet
Homework: p. 300 practice #2. P. 300 #2, 4, 6-8