Question

Complete combustion of 4.20 g of a hydrocarbon produced 12.9 g of CO2 and 6.15 g of H2O. What is the empirical formula for the hydrocarbon?

Answer 1

We calculate first the number of moles of CO2 and H2O produced by dividing the given masses by the molar masses of CO2 and H2O.
 moles CO2 = (12.9 g CO2) x (1 mole CO2 / 12 g CO2) = 1.075 moles.
 moles H2O = (6.15 g H2O) x (1 mole H2O / 18 g H2O) = 0.36 moles
Then, we count the number of C, H, and O moles. This gives us 1.075 moles C, 2.5 moles O and 0.72 moles H. The empirical formula is,
                             C1.075H0.72O2.5
Simplifying, 
                             C4H3O10