Complete combustion of 7.40 g of a hydrocarbon produced 22.8 g of CO2 and 10.5 g of H2O. What is the empirical formula for the hydrocarbon?

Answer 1

To find the empirical formula we want to first find moles of each element present.

moles C = 22.8 g CO2 x 1 mol CO2/44 g x 1 mol C/mol CO2 = 0.518 moles of C

moles H = 10.5 g H2O x 1 mol H2O/18 g x 2 mol H/mol H2O = 1.17 moles of H

Since the problem states that the compound being combusted is a HYDROCARBON, it should contain only C and H (carbon and hydrogen) and no other elements. That being the case, to find the empirical formula, we divide both by the lowest value to get...

C = 0.518/0.518 = 1

H = 1.17/0.518 = 2.25

And then to get whole numbers, we multiply both by 4 to get..

C = 1x4 = 4

H = 2.25x4 = 9

Empirical formula = C₄H₉

NOTE: If the compound actually does contain oxygen (O), then it isn't strictly a hydrocarbon, but you'd have to find masses of C and H from the moles, then subtract that mass from 7.40 g to get grams of O. Convert that to moles of O and proceed as above to find whole number moles.