Determine the empirical formula and the molecular formula for Ibuprofen.

Question

buprofen, the pain reliever in Advil and Motrin contains 75.69% C, 8.80% H, and 15.51% O by mass. The molecular mass is 206.3 g/mol. Determine the empirical formula and the molecular formula for Ibuprofen.

Answer 1

When given the % composition, you can assume 100 g of unknown material, and then the % is the same as grams. We then find moles of each element and then the empirical formula.

moles C =75.69 g C x 1 mol / 12.01 g = 6.30 moles C

moles H = 8.80 g H x 1 mol / 1.01 g = 8.71 moles H

moles O = 15.51 g O x 1mol / 16 g = 0.969 moles O

We want to get whole numbers, so we start by dividing all values by 0.969 to get...

moles C = 6.30 / 0.969 = 6.50 mols C

moles H = 8.71 / 0.969 = 8.99 mols H

moles O = 0.969 / 0.969 = 1 mol O

We can now simply multiply all values by 2 to get...

moles C = 13

moles H = 18

moles O = 2

Empirical formula = C₁₃H₁₈O₂

molar mass = 13x12 + 18x1 + 2x16 = 156 + 18 + 32 = 206 g/mol

To find molecular formula, divide molar mass by molar mass of empirical...

206.3 / 206 = 1 empirical formula in the molecular formula

Molecular formula = C₁₃H₁₈O₂