The heat of combustion of `CH_(4)(g), C_(2)H_(6)(g)` and `H_(2)(g)` are `890.3, 1559,7` and `285.9 kJ mol^(-1)`, respectively. Which of these fuels is most efficient?

Explanation

Fuel efficiency `eta_(CH_(4)) =(890.3)/(16) = ("Heat evolved")/("Molecular weight")`
`eta_(C_(2)H_(6)) = (1559.7)/(30) = 51.99 kJ`
and `eta_(H_(2)) =(285.9)/(2) = 142.95 kJ`
Hence, `H_(2)(g)` is the best fuel.