A hydrogen gas concentration cell $Pt ∣ H X_{2} (p X_{1}) ∣ HCl (aq) ∣ H X_{2} (p X_{2}) ∣ Pt$ has both electrodes dipped in the same HCl solution, with $p_{1} > p_{2}$ . At 298 K, which of the following statements are correct?

Answer: A, B, C

Oxidation

H X_{2} 2 H X^{+} + 2 e X^{-}

is favoured where

H X_{2}

pressure is higher, so the

p_{1}

electrode is the anode (A). Overall change is

H X_{2} (p_{1}) \to H X_{2} (p_{2})

;

H X^{+}

cancels, so EMF is independent of

[HCl]

(B).

E = \frac{0.0591}{2} lo g \frac{p _{1}}{p _{2}}

, which depends only on the ratio, so doubling both pressures changes nothing (C). Statement (D) uses

n = 1

instead of

n = 2

and is wrong.