The standard enthalpies of combustion at 298 K are -393.5 for graphite, -285.8 for H2(g) and -2220.0 kJ mol^-1 for C3H8(g) (products CO2(g) and H2O(l)). The standard enthalpy of formation of C3H8(g) is

JEE Advanced levelSingle correct optionDifficulty 3 of 3td152

The standard enthalpies of combustion at $298 K$ are $- 393.5$ for graphite, $- 285.8$ for $H X_{2} (g)$ and $- 2220.0 kJ mo l^{- 1}$ for $C X_{3} H X_{8} (g)$ (products $CO X_{2} (g)$ and $H X_{2} O (l)$ ). The standard enthalpy of formation of $C X_{3} H X_{8} (g)$ is

3 C + 4 H X_{2} C X_{3} H X_{8}

Δ_{f} H = \sum Δ_{c} H (reactants) - \sum Δ_{c} H (products) = 3 (- 393.5) + 4 (- 285.8) - (- 2220.0) = - 1180.5 - 1143.2 + 2220.0 = - 103.7 kJ mo l^{- 1}

. The wrong sign gives

103.7

; ignoring the Hess cycle sign for propane gives

- 4543.7

; omitting coefficients gives

1540.7