Determine the empirical & molecular formula of a compound...

Question

with a molar mass of 90.04g/mol that contains 26.7% C, 2.20% H, and 71.1% O.

Answer 1

The idea is to find the moles of each atom and then convert them to whole numbers.

If we assume that we have 100 g of the unknown, then the %'s become grams.

moles C = 26.7 g C x 1 mol C/12 g = 2.225 moles C

moles H = 2.20 g H 1 mol H /1 g = 2.20 moles H

moles O = 71.1 g O x 1 mol O/16 g = 4.44 moles O

Divide all by the smallest value to get whole numbers:

moles C = 2.225/2.20 = 1 mole C

moles H = 2.20/2.20 = 1 mol H

moles O = 4.44/2.20 = 2 mol O

Empirical formula = CHO₂

To find the molecular formula, we find how many of the empirical fit into the molar mass of the molecular.

molar mass empirical = 12 + 1 + 2x16 = 12 + 1 + 32 = 45 g/mol

90.04 g/ 45 g = 2 so it takes 2 empirical to make 1 molecular

Molecular formula = C₂H₂O₄