Empirical formula

Question

30.50g of an organic compound was combusted in oxygen to form 69.42 g of carbon dioxide and 28.40 g of water. In addition it was found that the compound contained 55.18% of oxygen. What is the empirical formula of the compound?

Answer 1

To find the empirical formula, we want to find the moles of each element at their lowest common ratio.

CxHyOz + O2 ==> CO2 + H2O

69.42 g CO2 x 1 mole CO2/44 g = 1.58 moles CO2 = 1.58 moles C

28.40 g H2O x 1 mol/18 g H2O x 2 mole H/mole H2O x 1 g H/mole H = 3.16 moles H

grams of C = 1.58 moles C x 12 g/mole = 18.96 g C

grams H = 3.16 moles H x 1 g/mole = 3.16 g H

grams O = 30.50 g - 18.96 g - 3.16 g = 8.38 g

moles O = 8.38 g x 1 mol/16 g = 0.524 moles O

Summary:

1.58 moles C

3.16 moles H

0.524 moles O

Dividing all by 0.524 in an attempt to get all whole numbers we have...

3.0 moles C

6.0 moles H

1.0 moles O

EMPIRICAL FORMULA = C₃H₆O