The standard heat of combustion of eicosane, C20H42 (s), a typical component of candle wax, is -1.332 x 104 kJ/mol (consider this number to be exact in your calculations), when it burns in pure oxygen, and the products are cooled to 25°C. The only products are CO2 (g) and H2O(l).

Question

Calculate ΔHf° for eicosane if ΔHf°(CO2 (g)) = -393.5 kJ/mol and ΔHfo(H2O(l)) = -285.9 kJ/mol.

Answer 1

C20H42 + 61/2 O2 ==> 20CO2 + 21H2O
dHrxn = (n*dHf CO2) + (n*dHf H2O(l)) - (n*dHf x) where x = C20H42
You're given Hf CO2 and Hf for liquid H2O and dHrxn, solve for x.

Answer 2

-1.332E4 = (21*-393.5)+(20*-285.9)-(n*dHf x)
-1.332E4 = -1.39815E4-(n*dHf x)
-6.615E2 = x

Answer 3

yes