A natural gas has the molar composition: methane 0.871, ethane 0.084, propane 0.023, CO_{2} 0.016 and H_{2}S 0.006. Molecular weights: CH_{4} 16.04, C_{2}H_{6} 30.07, C_{3}H_{8} 44.09, CO_{2} 44.01, H_{2}S 34.08; air 28.97. Calculate (a) the molecular weight of the gas (kg/kmol) and (b) the gas gravity (specific gravity relative to air).