Question

The EMF of the cell $M|M^{n+}\left(0.02M\right)||H^{+}\left(1M\right)|H_2\left(g\right)\left(1atm\right),Pt$ at $25{}^{o}C$ is $0.81V$. The valency of the metal if the standard oxidation potential of the metal is $0.76V$?
($\log 0.02=-1.7$)

• A. $5$
• B. $2$
• C. $4$
• D. $3$

Answer 1

The correct option is B $2$

$A:M-ne⟶M^{n+}{E}^o=+0.76$
$C:n{H}^{+}+ne⟶\frac{n}{2}{H}_2\left(g\right)E^o=0$
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$M+n{H}^{+}⟶M^{n+}+\frac{n}{2}{H}_2\left(g\right)E^o=+0.76$

We know,
$E_{cell}=0.81=+0.76-\frac{0.0591}{n}\log \frac{\left[M^{n+}\right]}{[H^{+}{]}^n}$
$\Rightarrow -\frac{0.0591}{n}\log \frac{0.02}{1^n}=0.81-0.76$
$\Rightarrow \frac{0.0591}{n}\times 1.7=0.81-0.76\Rightarrow n=\frac{0.0591\times 1.7}{0.05}\approx 2$