When a 8.37-g mixture of methane, CH4, and ethane, C2H6, is burned in oxygen at constant pressure, 445.8 kJ of heat is liberated. What is the percentage by mass of CH4 in the mixture? The standard enthalpies of combustion for CH4 and C2H6 are -890.3 kJ mol-1 and -1569.7 kJ mol-1, respectively.