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Calculating Empirical Formula:
1. If given %, assume 100g sample.
2. Convert mass of each element to moles.
3. Divide each of these numbers by the smallest number to force one of them to = 1.
4. If necessary, multiply by the smallest number possible to make each a whole number.
5. These whole numbers are the subscripts in the empirical formula called mole ratio.

Example: Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?
1. Treat % as mass, and convert grams to moles

2. Divide each value of moles by the smallest of the values.

Carbon:

Hydrogen:

Oxygen:

3. Multiply each number by an integer to obtain all whole numbers.

Carbon: 1.50 x 2 = 3
Hydrogen: 2.5 x 2 = 5
Oxygen: 1.50 x 2 = 3
Empirical Formula: C3H5O2





Calculating Molecular Formula:
1. Find molar mass of empirical formula.
2. Divide experimentally determined molar mass (given), by molar mass of empirical formula.

3. Has to equal a whole number “n”
4. Multiply each subscript by n.

Example: The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g
2. Divide the molecular mass by the mass given by the empirical formula.

3. Multiply the empirical formula by this number to get the molecular formula.

(C3H5O2) x 2 = C6H10O4

1. Determine the molecular formula (true formula) for the compounds below from the percent by mass given for each element.

a. A gas with the empirical formula of NO2 is found to have a molar mass of 92 g/mol.
b. A gas that has the empirical formula of CH2 is determined to have a molar mass of 70 g/mol.
c. A compound with a molar mass of 74 g/mol has an elemental analysis of 64.9% carbon, 21.6% oxygen, and 13.5% hydrogen.
d. A compound with an empirical formula of CH has a molecular weight of 78 g/mol.

nitrogen per mole of compound. The
elemental analysis shows 49.48% car
bon, 28.85% nitrogen, 16.48% oxygen, and 5.19%
of hydrogen.
Ans: