Question

An aqueous solution of $NaCl$ on electrolysis gives $H _{2}(g), Cl _{2}(g)$ and $NaOH$ according to the reaction.
$2 Cl ^{-}(a q)+2 H _{2} O \rightleftharpoons 2 OH (a q)+ H _{2}(g)+ Cl _{2}(g)$
A direct current of $25 A$ with a current efficiency of $62 \%$ is passed through $20 L$ of $NaCl$ solution ( $20 \%$ by weight). Write down the reactions taking place at the anode and cathode. How long will it take to produce $1 kg$ of $Cl _{2}$ ? What will be the molarity of the solution with respect to hydroxide ion? (Assume no loss due to evaporation)

Answer 1

At anode $ 2 Cl ^{-} \longrightarrow Cl _{2}+2 e^{-}$
At cathode $2 H _{2} O +2 e^{-} \longrightarrow H _{2}+2 OH ^{-}$
$1 \,kg \,Cl _{2}=\frac{1000}{35.5}$ equivalent of $Cl _{2}=28.17$ equivalent
$\Rightarrow$ Theoretical electricity requirement $=28.17 F$
$\because$ Efficiency is only $62 \%$
$\therefore$ Electricity requirement (experimental)
$=\frac{28.17 \times 100}{62} F =45.44 F$
$\Rightarrow 45.44 \times 96500=25 t$ (in second)
$\Rightarrow t=48.72\, h$
Also, gram equivalent of $HO ^{-}$produced $=28.17$
$\Rightarrow$ Molarity of $ HO ^{-}=\frac{28.17}{20}=1.4085 \,M$