Q.A compound on analysis gave Na = 14.31%, S = 9.97%, H = 6.22% and O = 69.5%. Calculate the molecular formula of the compound, if all the hydrogen in the compound is present in combination with oxygen as water of crystallisation. (Molecular mass of the compound is 322.)

The empirical formula of a compound is written using the simplest whole-number ratio of its different atoms; the molecular formula is written using the actual number of each atom in one molecule. The molecular formula is always a whole-number multiple of the empirical formula (e.g. acetic acid's molecular formula C₂H₄O₂ is 2 × its empirical formula CH₂O).

Finding the empirical formula from elemental analysis (mass percentages):

1. Treat the percentage composition as grams out of a 100 g sample.
2. Divide each element's mass by its atomic mass, to get relative moles.
3. Divide every value by the smallest one, to get the simplest ratio.
4. If the ratio still has fractions, scale all values up by the smallest number that clears them.

Finding the molecular formula from the empirical formula: once you know the compound's actual molar mass (from a separate experiment, e.g. vapour density or a colligative-property measurement), compute the whole-number multiplier:

n = (molar mass of the compound) ÷ (calculated empirical-formula mass)

and the molecular formula is (empirical formula) × n. …