Practice Problems: Empirical and Molecular Formulas
1. Benzene, a non-polar solvent used for many applications in industry, and a major component
in many organic compounds is 92.3% Carbon and 7.8 % Hydrogen.
a. Find Benzene’s empirical formula.
Since you are given percentages, assume you have a 100 gram sample of benzene.
Therefore, you have 92.3 g of Carbon and 7.8 g of Hydrogen.
92.3 g C x 1 mole C/12.01 g C = 7.69 moles C
7.8 g H x 1 mole H/1.0079 g H = 7.74 moles H
This gives you C7.69H7.74, divide each subscript by the lowest value (7.69).
The empirical formula of benzene is CH.
b. Find the Molecular formula of benzene if the entire formula mass is 78.12 g/mol
Since benzene’s empirical formula is CH, its empirical mass is 13.0179 g. Divide the
molecular mass by the empirical mass to determine how many times larger the molecular
formula is. 78.12/13.0179 = roughly 6. This means the molecular formula of benzene is
C6H6.
2. An unknown sugar is found to have a formula mass of 180.18 g/mol. The sugar contains:
40.0 % C, 6.7 % H and 53.3 % O.
a. Find the empirical and molecular formula of this sugar.
Again, assume you have 100 g sample of the sugar:
40.0 g C x 1 mole C/12.01 g = 3.33 moles C
6.7 g H x 1 mole H/1.0079 g H = 6.65 moles H
53.3 g O x 1 mole/15.999 g O = 3.33 moles O
C3.33H6.65O3.33 – divide by lowest mole value to get an empirical formula of CH2O. The
empirical mass off this compound is 30.02 g/mol. Dividing the molecular mass by the
empirical mass, you calculate that the molecular formula is 6x the empirical, so its
C6H12O6.
b. What’s its name?
You wouldn’t have to know this, but its glucose.
3. Tryptophan – the chemical in turkey that is believed to make you sleepy – has the empirical
formula C11H12N2O2. Find the molecular formula if the formula mass is 204.25 g/mol.
The empirical mass is about 204.22 g/mol. It could be even closer depending on how many
significant figures you use in your calculations. Since that’s virtually the same number as
the molecular mass, the empirical and molecular formula are the same value.
4. The caffeine in Diet Coke is made of 49.48 % C, 5.19% H, 16.48% O and 28.8% N. Find the
molecular mass of Caffeine if its overall molecular mass is 194.22 g/mol
Assume a 100 g sample:
49.48 g C x 1 mole C/12.01 g C = 4.12 moles C
5.19 g H x 1 mole H/1.0079 g H = 5.15 moles H
16.48 g O x 1 mole O/15.999 g O = 1.03 moles O
28.8 g N x 1 mole N/ 14.007 g N = 2.06 moles N
C4.12H5.15O1.03N2.06. Divide by the lowest number, giving you an empirical formula of
C4H5ON2. The empirical mass is 97.09 g/mol. The molecular mass is 2x larger, meaning
the molecular formula is C8H10O2N4.
5. Hydrogen peroxide is 5.93 % H and 94.07 % O. Find the formula of hydrogen peroxide given
it has an overall formula mass of 34 g/mol.
Assume 100 gram sample:
5.93 g H x 1 mol H/1.0079 g H = 5.88 moles H
94.07 g O x 1 mol O/15.999 g O = 5.88 moles O
Since both mole values are the same, the empirical formula is HO. Its mass is 17.01 g/mol.
The molecular mass is 2x the empirical value, so the molecular formula is H2O2.
6. A strong oxidizing agent and rocket propellant has a % composition of 30.43% N and 69.57 %
O. Find the molecular formula if its formula mass is 92.0 g/mol.
Assume 100 g sample:
30.43 g N x 1 mole N/ 14.007 g N = 2.17 moles N
69.57 g O x 1 mole O/15.999 g O = 4.35 moles O
N2.17O4.35 Divide each mole value by 2.17
NO2 is the empirical formula. Its mass is 46.005 g. Since the formula (molecular) mass is
2x the empirical mass, the molecular mass is N2O4.