Empiricial and molecular formula

Question

An unknown sample contains 84.415 % carbon, and 15.585 % hydrogen. It has a molar mass of 142.3g/mol. Determine the empirical formula and the molecular formula

Answer 1

When given the % composition, we can assume we have 100 g of unknown, and then the % becomes grams.

We then calculate moles of C and moles of H to find the empirical formula.

mols C = 84.415 g C x 1 mol C / 12.011 g = 7.028 mols C

mols H = 15.585 g H x 1 mol H / 1.008 g = 15.46 mols H

Divide both by 7.028 in an attempt to get whole numbers

7.028 / 7.028 = 1.00 mol C

15.46 / 7.028 = 2.2 mols H

multiply both by 5 to get whole numbers

1.00 mol C x 5 = 5 mols C

2.2 mol H x 5 = 11 mols H

Empirical formula = C₅H₁₁

To find the molecular formula, we need to see how many of the empirical formula molecular weights fit into the molecular weight of the unknown.

molecular wt C₅H₁₁ = 71 g/ mol

142.3 / 71 = 2

Molecular formula has 2 empirical formulae in it

Molecular formula = C₁₀H₂₂