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 C12.01g C=7.130 mol C
Moles of H=14.37g H×1 mol H1.008g H=14.26 mol H
Moles of CMoles of H=7.130mol14.26mol=12.000≈12
The empirical formula is CH2.
