What is the empirical formula of a phosphoric acid that contains 0.3086 g of hydrogen 3.161 g of phosphorus and 6.531 g of oxygen?

moles H = 0.3086 H x 1 mol/1 g = 0.3086 moles H
moles P = 3.161 g x 1 mol/31 g = 0.101 moles P
moles O = 6.541 g x 1 mol/16 g = 0.41 moles O
divide all by 0.101 to get 3.0 moles H, 1 mole P and 4 moles O
Empirical formula = H3PO4