JEE Advanced 2025 — Paper 1 — CHEM-III solution

JEE Advanced2025Paper 1CHEM-III

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 $^{- 1}$ ; Atomic mass (in amu): H = 1, O = 16

Your answer

Solution

Electrolysis reaction: $H_{2} O (l) \to H_{2} (g) + \frac{1}{2} O_{2} (g)$ .

Moles of water: $\frac{144}{18} = 8$ mol. So 8 mol H $_{2}$ (g) and 4 mol O $_{2}$ (g) are released, totaling $Δ n_{g} = 12$ mol of gas formed from a liquid (taken as having negligible volume).

Constant pressure expansion work done by the system:

$∣ W ∣ = Δ n_{g} \cdot R T = 12 \times 8.3 \times 300 = 29880$ J $= 29.88$ kJ.

$∴$ Expansion work done $= 29.88$ kJ.

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