Many portable gas heaters and grills use propane, C3H8(g).

Using enthalpies of formation, calculate the quantity of heat produced when 19.0g of propane is completely combusted in air under standard conditions. Assume that liquid water is forming.

3 answers

C3H8 + 5O2 ==> 3CO2 + 4H2O

dHrxn = (n*dHf products) - (n*dhf reactants)
Then convert dHrxn from 44g (1 mol in the equation above) to 19.0g.
? = dHrxn x 19.0/44
If you carry the sign over from the first calculation you will get the correct sign for the smaller amount.
-1001.7
-957