Chemistry
Empirical & Molecular Formulas
Molecular formula: a formula showing the types and numbers of atoms combined in a single
molecule of a molecular compound. It is a whole number multiple of the empirical formula.
The relationship between a compound’s empirical and molecular formulas can be written as:
x(empirical formula) = molecular formula
also
x(empirical formula mass) = molecular formula mass
1. To determine the molecular formula of a compound, you must know the compound’s formula
mass.
2. Divide the molecular mass by the empirical formula mass to determine the whole number
multiple (x).
You may have to find the empirical formula in order to obtain the empirical formula mass
1. A 5.438 gram sample, was found to contain 2.549 grams of iron, 1.947 grams of oxygen,
and 0.9424 grams of phosphorus. What is its empirical formula?
2. A certain compound containing only phosphorus and oxygen is 56.3 % P (by mass)
determine the empirical formula of this compound.
3. A compound with an empirical formula of CHCl2 is found to have a molar mass of 167.80
g/mol. What is its molecular formula?
4. A compound contains 63.11 % C, 12.36 % H, and 24.53 % N.
A. What is its empirical formula?
B. If the experimental molar mass is 114.2 g/mol, what is the molecular formula?
5. A certain carbohydrate compound (containing only C, H and 0) is 40.0% C, 6.72% H, and
53.3% O by mass. The experimentally determined molecular mass is 180 g/mol. What is the
empirical and molecular formula for this carbohydrate?
6. A certain hydrocarbon compound (containing only C and H) is 83.6% C and 16.4% H by
mass. The experimentally determined molecular mass is 86.2 amu. What is the empirical
and molecular formula for this hydrocarbon?
7. What is the molecular formula for a compound with an empirical formula of C2OH4 and a
molar mass of 88 grams per mol?
8. A component of protein called serine has an approximate molar mass of 100.0 g/mole. If the
percent composition is as follows, what are the empirical and molecular formulas for serine?
C=34.95%; H=6.844%; O=46.56%; N=13.59%
9. A compound contains 75.46% carbon, 4.43% hydrogen, and 20.11% oxygen, by mass. It
has a molar mass of 318.31 g/mol. What is the molecular formula?
10. A compound is 92.2% Carbon and 7.76% Hydrogen. The formula mass of the compound is
78.1 g/mol. Determine the empirical formula and molecular formula of the compound.
Challenge Question
11. Aniline, a starting material for urethane plastic foams, consists of C, H, and N. Combustion
of such products yields CO2, H2O, and N2 as products. If the combustion of 9.71g of aniline
yields 6.63g H2O and 1.46 g N2, what is its empirical formula?
12. The molar mass of aniline is 93g/mol. What is its molecular formula?
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