A gaseous hydrocarbon gives upon combustion, 0.72 g of water and 3.08 g of CO2.  The empirical formula of the hydrocarbon is

18 g H2O  contain  2  g  H.∴   0.72  g H2O  contain  0.08  g H44 g CO2 contain 0.84  g C.∴  3.08 g  CO2 contain  0.84  g C.∴  C:H  = 0.8412  :  0.081=0.07  :  0.08  = 7  : 8∴    Empirical  formula  =  C7H8