Explain how to determine the molecular formula of a compound?

To get the molecular formula, you must either know the molecular mass or do an experiment to find it.

Example

An unknown compound contains 85.63 % C and 14.37 % H. Its experimental molar mass is 56 g/mol. What is its molecular formula?

Solution

Assume we have 100 g of the compound. Then we have 85.63 g of C and 14.37 g of H.

Moles of C=85.63g C * 1 mol C 12.01g C = 7.130 mol C

Moles of H=14.37g H * 1 mol H 1.008g H = 14.26 mol H

Moles of CMoles of H = 7.130mol 14.26mol = 12.000 ≈ 12

The empirical formula is CH2.