Empirical and Molecular Formula

Question

A compound containing only phosphorous and oxygen was found to have 43.64 % phosphorous by mass and 56.36 % oxygen by mass. The molecular formula has a molar mass of 141.94 g/mol.  Determine the empirical formula and the molecular formula of the compound.

Answer 1

Assume you have 100 g of the material. This way you can use the % as grams and then convert to moles.

Moles P = 43.64 g P x 1 mol/30.97g = 1.409 mol P

Moles O = 56.36 g O x 1 mol/16 g = 3.52 mol O

Divide both by 1.409 to try to get whole numbers

Moles P = 1.409/1.409 = 1.0 mol

Moles O = 3.53/1.409 = 2.5 mol

multiple both by 2 to get

moles P = 2

moles O = 5

EMPIRICAL FORMULA = P₂O₅

Molar mass of P₂O₅ = 142 g/mole

Since this is the same as the molar mass of the molecular formula the

MOLECULAR FORMULA = P₂O₅