Question

Salt hydrolysis describes the interaction of water and cation/anion or both of a salt. The pH of the solution is affected by this interaction. The cations of strong acids do not hydrolyse and therefore the solutions of salts formed by strong acids and strong bases are neutral, i.e. their pH is 7, whereas the solutions of salts formed by strong bases and weak acids are alkaline, i.e. their pH>7. While the solutions of salts formed by strong acids and weak bases are acidic i.e. pH<7.

List-I contains questions related to salt hydrolysis given above and List-II contains their answers

$\begin{array}{ccc}\text{List - I (Salt solution)}& \text{List - II (pH)}& \\ \text{(I) 0.1 M}KCN& \left(P\right)7& \\ \text{(II) 0.1 M}PhN{H}_{3}Cl& \text{(Q) 6.5+}\frac{1}{2}p{K}_a& \\ \text{(III) 0.1 M}KCl& \text{(R) 7.5}-\frac{1}{2}p{K}_b& \\ \text{(IV) 0.1 M}C{H}_{3}COON{H}_4(K_a=K_b)& \text{(S)}7.5+\frac{1}{2}p{K}_a-\frac{1}{2}p{K}_b& \\ & \text{(T)}7.5-\frac{1}{2}p{K}_a-\frac{1}{2}p{K}_b& \\ & \text{(U)}7.5-\frac{1}{2}p{K}_a+\frac{1}{2}p{K}_b& \end{array}$

Which of the following options has the correct combination considering Lists-I and List-II?

• A. $\left(I\right),\left(S\right)$ and $\left(III\right),\left(P\right)$
• B. $\left(I\right),\left(Q\right)$ and $\left(III\right),\left(P\right)$
• C. $\left(I\right),\left(U\right)$ and $\left(III\right),\left(R\right)$
• D. $\left(I\right),\left(T\right)$ and $\left(III\right),\left(S\right)$

Answer 1

The correct option is B $\left(I\right),\left(Q\right)$ and $\left(III\right),\left(P\right)$

(I) It is a salt of a weak acid $\left(HCN\right)$ and a strong base $\left(KOH\right)$.
$pH=\frac{1}{2}(p{K}_w+p{K}_a+logC)$
$=\frac{1}{2}(14+p{K}_a+log{10}^{-1})$
$=6.5+\frac{1}{2}p{K}_a$

(III) It is a salt of strong acid $\left(HCl\right)$ and strong base $\left(KOH\right)$. So the $pH=7$