Question

Two litre solution of a buffer mixture containing 1.0 M $Na{H}_{2}P{O}_4$ and 1.0M $N{a}_{2}HP{O}_4$ is placed in two compartments (one litre in each) of an electrolytic cell. The platinum electrodes are inserted in each compartment and 1.25 ampere current is passed for 212 minute. Assuming electrolysis of water only at each compartment. What will be pH in each compartment after passage of above charge? $p{K}_a$ for $H_{2}P{O}_4^{-}$ = 2.15.

• A. $5.6$
• B. $3.2$
• C. $4.7$
• D. $9.8$

Answer 1

The correct option is A $5.6$

$Na{H}_{2}P{O}_4\rightleftharpoons N{a}_{2}HP{O}_4$

Using Handerson equation:
$pH=p{K}_a+log\frac{\left[Salt\right]}{\left[Acid\right]}$

$pH=2.15+log1=2.15$

$4H^{+}+4e^{-}+O_2\to 2H_{2}O$

$96500C$ will be used for 0.5 mole $H_{2}O$ formation