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JEE Advanced 2025 · Paper 1, Chemistry question 11

Considering ideal gas behavior, the expansion work done (in kJ) when 144 g of water is electrolyzed completely under constant pressure at 300 K is _____.

Use: Universal gas constant (R) =8.3 J K−1 mol−1= 8.3\ \mathrm{J\,K^{-1}\,mol^{-1}}; Atomic mass (in amu): H = 1, O = 16

Answer and worked solution

Official answer: -29.95

Gas formed. 144 g of water is 8 mol. From 2 HX2O→2 HX2+OX2\ce{2H2O -> 2H2 + O2} this gives 8 mol HX2\ce{H2} and 4 mol OX2\ce{O2}, so Δng=12\Delta n_g=12.

Work at constant pressure.
w=−PΔV=−ΔngRT=−12×8.3×300=−29880 J≈−29.88 kJ.w=-P\Delta V=-\Delta n_gRT=-12\times8.3\times300=-29880\ \text{J}\approx-29.88\ \text{kJ}.

The work done by the system is +29.88 kJ, and the key accepts either sign.

Topics:First law and enthalpyStoichiometry
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