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

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  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.

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  2. -1.332E4 = (21*-393.5)+(20*-285.9)-(n*dHf x)
    -1.332E4 = -1.39815E4-(n*dHf x)
    -6.615E2 = x

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  3. yes

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