A compound is found to contain 3.1% Hydrogen, 65.3% Oxygen and 31.6% Phosphorus. Work out its empirical formula.

To answer this question, we need to know the relative number of atoms of each element present. This can be calculated by dividing the amount of an element present in the sample by the relative masses of that element. Relative masses: Hydrogen = 1.0 Oxygen = 16.0 Phosphorus = 30.0. Relative number of atoms = (%composition)/(relative atomic mass) Hydrogen = 3.1 Oxygen = 4.08 Phosphorus = 1.05. This gives us the relative ratios of the elements: 3H's, 4O's, 1P. Therefore the empirical formula is H₃PO₄.