Electrochemistry and Ionic Equilibria Questions and Answers

Answer: C

Permanganate can oxidise Zn and Ag.

An oxidant can oxidise a metal when its reduction potential is higher than that of the metal ion/metal couple. Permanganate's couple is +1.51 V. The silver couple is +0.7996 V and the zinc couple -0.761 V, both lower, so MnO₄^- can oxidise both metals:

\[ \begin{aligned} E^{o}_{cell}(\text{Ag}) &= 1.51 - 0.80 = +0.71\ \text{V} \\ E^{o}_{cell}(\text{Zn}) &= 1.51 + 0.76 = +2.27\ \text{V} \end{aligned} \]

The gold couple, +1.692 V, is above +1.51 V, so for gold the cell potential would be negative and permanganate cannot oxidise it. Every option containing Au is wrong.

Answer: D

The pH is close to 1.70.

\[ \begin{aligned} n(\text{H}^{+}) &= 0.800 \times 0.1 = 0.080\ \text{mol} \\ n(\text{OH}^{-}) &= 0.200 \times 0.3 = 0.060\ \text{mol} \\ \text{excess H}^{+} &= 0.020\ \text{mol in } 1.0\ \text{L} \end{aligned} \]

\[ \text{pH} = -\log(0.020) = 1.70 \]

The original 0.1 M HCl had pH 1.00, so 1.10 is too acidic once 0.060 mol of the acid has been neutralised. 13.3 would mean excess base, and 6.90 near-exact neutralisation, but the acid is in excess by 0.020 mol in the combined 1 L.