Percent composition and empirical formula

Question

A compound consists of 85.7% carbon and 14.3% hydrogen. Determine its empirical formula?

if the compound in the question above has 4 molar mass of 392g/mol, what is its molecular formula?

Answer 1

Assume 100 g of the unknown. Then…

85.7 g C x1 mol C/12 g = 7.14 mol C

14.3 g H x 1 mol H / 1 g = 14.3 mol H

Divide both by the lowest (7.14) to get whole numbers

mols C = 7.14/7.14 = 1

mols H = 14.3/7.1 = 2

Empirical formula = CH₂

Molar mass = 12 + 2 = 14 g/mol

Molecular formula = 392/14 = 28 empiricals

Molecular formula = C₂₈H₅₆