Question

A sample of natural gas is 80.0% CH4 and 20.0% C2H6 by mass. What is the heat from the combustion of 1.00 g of this mixture?

Answer 1

add heats from each part
heat from methane: .80*Hc (you may have to convert Hc to joules/gram)
heat from Ethane: .20*Hc ibid.