How to Find Empirical Formula

Question

An unknown compound (containing only carbon and hydrogen) combusts and produces 1.60 g CO₂ and 0.819 g H₂O. What is the empirical formula of the compound?

Answer 1

Since the unknown contains only C and H, we can calculate moles of C from the CO₂and moles of H from the H₂O.

1.6 g CO₂ x 1 mol CO₂/44 g = 0.0363 moles CO₂ x 1 mol C/mole CO₂ = 0.0363 moles C

0.819 g H₂O x 1 mole/18 g = 0.0455 moles H₂O x 2 moles H/mol H₂O = 0.091 moles H

Dividing both by 0.0363 to get a whole number for moles of C, we get 1 mole C and 2.5 moles of H. If we now multiply by 2, we get

2 moles C and 5 moles of H. Empirical formula thus is

C₂H₅

Answer 2

Carbon dioxide has a molecular weight of 44.01 grams per mole.

Water has a molecular weight of 18.015 grams per mole.

Divide the number of grams by the molecular weight to get the number of moles

1.60/44.01 = 0.036355 moles, or 36.355 millimoles carbon dioxide. Each carbon dioxide molecule contains one carbon atom, so this represents 36.355 millimoles of carbon

0.819/18.015 = 0.045462 moles, or 45.462 millimoles water. Each water molecule contains two hydrogens, so this represents 90.924 moles of hydrogen.

The ratio of hydrogen to carbon is 90.924/36.355 = 2.50. This means there are 2.5 hydrogen atoms to each carbon atom. Multiply by 2 to get a ratio of 5 hydrogens to 2 carbons. The empirical formula is therefore

C2H5

Answer 3

C2H5 (See video for notes and steps).