The molecular formula is either the same as the empirical formula or a multiple of it.Given a molecular formula, you can always write the empirical formula just by looking to see
whether the numbers of atoms are already in the smallest whole number ratio. If not, you can factor
out the common factor among them. C 2 H 4 is not an empirical formula because you can factor out a
two from the subscripts to get CH 2 . CH 4 , on the other hand, is already an empirical formula. If you
are given an empirical formula, however, you need to know the molecular weight (or molar mass) of
the compound to find out the actual molecular formula.
Example: A compound with the empirical formula CH 2 O has a weight of 180 g/mol. What is the
molecular formula?
Solution: Let us first find what the formula weight is from the empirical formula: 1 × mass of
carbon atom + 2 × mass of hydrogen atom + 1 × mass of oxygen atom = (1 × 12 + 2 ×
1 + 1 × 16) g/mol = 30 g/mol
The actual molecular weight is 6 times this; therefore the molecular formula must
be 6 times the empirical formula: C 6 H 12 O 6.PERCENT COMPOSITION
The percent composition by mass of an element is the weight percent of the element in a specific
compound. To determine the percent composition of an element X in a compound, the following
formula is used:
The percent composition of an element may be determined using either the empirical or molecular
formula.
Example: What is the percent composition of chromium in K 2 Cr 2 O 7?
Solution: The formula weight of K 2 Cr 2 O 7 is:
2(39 g/mol) + 2(52 g/mol) + 7(16 g/mol) = 294 g/mol
% composition of