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?

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