Problem: A gaseous fuel mixture contains 23.1% methane (CH4), 41.1% ethane (C2H6) and the rest propane (C3H8) by volume.When the fuel mixture contained in a 1.53 L tank, stored at 751 mmHg and 298 K, undergoes complete combustion, how much heat is emitted? (Assume that the water produced by the combustion is in the gaseous state.)

We are asked how much heat is emitted

Since the gas are under the same conditions, the volume percent is the same as the mole percent. 

Calculate total moles: 

$$\begin{gathered} \boxed{\mathbf{PV}\mathbf{=}\mathbf{ }\mathbf{nRT}} \\ \mathbf{n}\mathbf{ }\mathbf{=}\mathbf{ }\frac{\mathbf{PV}}{\mathbf{RT}} \\ \mathbf{n}\mathbf{ }\mathbf{=}\mathbf{ }\frac{(751 \cancel{\mathrm{mmHg}}\times {\displaystyle \frac{1 \cancel{\mathrm{atm}}}{760 \cancel{\mathrm{mmHg}}}})(1.53 \cancel{L})}{(0.08206 {\displaystyle \frac{ \cancel{L}- \cancel{\mathrm{atm}}}{\mathrm{mol}-\cancel{K}}})(298 \cancel{K})} \end{gathered}$$

n = 0.0617 mol

Calculate moles of each: 

$$\begin{gathered} \mathbf{mol}\mathbf{ }{\mathbf{CH}}_{\mathbf{4}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{0}\mathbf{.}\mathbf{231}(0.0617 \mathrm{mol})\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{0}\mathbf{.}\mathbf{014}\mathbf{ }\mathbf{mol}\mathbf{ }{\mathbf{CH}}_{\mathbf{4}}\mathbf{ } \\ \mathbf{mol}\mathbf{ }{\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{6}}\mathbf{ }\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{411}(0.0617 \mathrm{mol})\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{0}\mathbf{.}\mathbf{025}\mathbf{ }\mathbf{mol}\mathbf{ }{\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{6}}\mathbf{ } \\ \mathbf{mol}\mathbf{ }{\mathbf{C}}_{\mathbf{3}}{\mathbf{H}}_{\mathbf{8}}\mathbf{ }\mathbf{=}(1-0.231-0.411)(0.0617 \mathrm{mol})\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{0}\mathbf{.}\mathbf{022}\mathbf{ }\mathbf{mol}\mathbf{ }{\mathbf{C}}_{\mathbf{3}}{\mathbf{H}}_{\mathbf{8}} \end{gathered}$$