Question

Chromium plating can involve the electrolysis of an electrolyte of an acidified mixture of chromic acid and chromium sulphate. If during electrolysis the article being plated increases in mass by 2.6 g and $0.6d{m}^3$ of oxygen are evolved at an inert anode, the oxidation state of chromiumions being discharged must be : (assuming atomic weight of Cr = 52 and 1 mole of gas at room temperature and pressure occupies a volume of 24 $d{m}^3$)

• A. $-1$
• B. $Zero$
• C. $+1$
• D. $+2$

Answer 1

Answer: (C)

$+1$

$4H^{\oplus }\to O_2+2H_{2}O+4e^{-}\Rightarrow 1mol\equiv 4F$
$\Rightarrow \frac{0.6}{24}mol\equiv 4\times \frac{0.6}{24}=0.1F$
$C{r}^{3+}+x{e}^{-}\to C{r}^{(3-x)\oplus }$
$\Rightarrow 1mol\equiv xF\Rightarrow \frac{2.6}{52}mol\equiv \frac{x}{20}F\Rightarrow x-2$
$\Rightarrow$ Oxidation state of $C{r}^{(3-2)}=C{r}^{+1}=+1$.