Question

$pH$ of $0.1 \,M$ solutions of $KX , KY$ and $KZ$ are $8,9$ and $10$ respectively. Thus order of acidic strength of $HX , HY$ and $HZ$ is

• A. $HX < HY < HZ$
• B. $HZ < HX < HY$
• C. $HY < HX < HZ$
• D. $HZ < HY < HX$

Answer 1

Answer: (D)

These are the salts of strong base with weak monobasic acid. Solution is basic due to hydrolysis of anion.

$X ^{-}+ H _{2} O \rightleftharpoons HX + OH ^{-}$

$pH =7+\frac{p K_{a}}{2}+\frac{\log C}{2}$

$=7+\frac{p K_{a}}{2}+\frac{\log \overset{.}{0}{1}}{2}=7+\frac{p K_{a}}{2}-0.5$

$\therefore pk _{ a }=2( pH -6.5)$

$p k_{a}( HX )=2(8-6.5)=3.0, K_{a}=10^{-3}$

$p k_{a}( HX )=2(9-6.5)=5.0, K_{a}=10^{-5}$

$p k_{a}( HZ )=2(10-6.5)=7.0, K_{a}=10^{-7}$

Thus, $HX$ is the strongest acid.

$\therefore HZ < \,HY <\, HY$