2019 H2 Chem P1 Q29 — Electrochemical cell Solution

The answer is B (1, 1, 1 mol dm\(^{-3}\)).

A standard electrode potential, \(E^\ominus\), is measured under standard conditions: 298 K, 1 bar for gases, and every aqueous species in the half-equation at 1 mol dm\(^{-3}\). The stoichiometric coefficients (2 for HOCl and \(\text{H}^+\)) do not change this: each species is simply at 1 mol dm\(^{-3}\).
Here HOCl, \(\text{H}^+\) and dissolved \(\text{Cl}_2\) are all in solution, so all three must be 1 mol dm\(^{-3}\): row B.

• A is wrong: \([\text{Cl}_2]\) = 0.5 mol dm\(^{-3}\) is not standard; it treats the concentrations as a 2 : 2 : 1 ratio scaled down instead of setting each to 1 mol dm\(^{-3}\).
• C is wrong: it sets [HOCl] and \([\text{H}^+]\) to 2 mol dm\(^{-3}\) from their coefficients in the equation; coefficients do not set standard concentrations.
• D is wrong: all three species are at 2 mol dm\(^{-3}\), not the standard 1 mol dm\(^{-3}\).