When the fuel mixture contained in a 1.49 l tank, stored at 750 mmhg and 298 k, undergoes complete combustion, how much heat is emitted? (assume that the water produced by the combustion is in the gaseous

Get to know first how many moles in the gas:n = pV/RT= (1.013*10^5*750/760) Pa *1.49*10^-3 m^3/(8.314 J/(molK)*298) n = 0.0601 moles. 
The combustion energies  are  889 kJ/mol (methane) and 2 220 kJ (propane) x = moles methane, y = moles propane 
x*889 + y*2220 = 778 x + y = 0.0601 x = 0.267784 moles = 0.267784*100/0.0601 = 44.6 % y = 0.243216 moles = 0.243216*100/0.0601 = 55.4 %