Question

Arrange the following solutions in the decreasing order of their $\mathrm{pOH}$:

$$\begin{gathered} \left(\mathrm{A}\right)0.01\mathrm{M}\mathrm{HCl} \\ \left(\mathrm{B}\right)0.01\mathrm{M}\mathrm{NaOH} \\ \left(\mathrm{C}\right)0.01\mathrm{M}{\mathrm{CH}}_3\mathrm{COONa} \\ \left(\mathrm{D}\right)0.01\mathrm{M}\mathrm{NaCl} \end{gathered}$$

• A. $\left(\mathrm{A}\right)>\left(\mathrm{C}\right)>\left(\mathrm{D}\right)>\left(\mathrm{B}\right)$
• B. $\left(\mathrm{B}\right)>\left(\mathrm{D}\right)>\left(\mathrm{C}\right)>\left(\mathrm{A}\right)$
• C. $\left(\mathrm{B}\right)>\left(\mathrm{C}\right)>\left(\mathrm{D}\right)>\left(\mathrm{A}\right)$
• D. $\left(\mathrm{A}\right)>\left(\mathrm{D}\right)>\left(\mathrm{C}\right)>\left(\mathrm{B}\right)$

Answer 1

The correct option is D

$\left(\mathrm{A}\right)>\left(\mathrm{D}\right)>\left(\mathrm{C}\right)>\left(\mathrm{B}\right)$

Explanation of the correct answer.

Formulae used - $\mathrm{pH}=-\log {\mathrm{H}}^{+}$

$\mathrm{pOH}=-\log {\mathrm{OH}}^{-}$

$\mathrm{pH}+\mathrm{pOH}=14$

Step 1 - Calculation of pH of $0.01\mathrm{M}$ HCl -

$0.01\mathrm{M}\mathrm{HCl}$contains ${10}^{-2}\mathrm{M}$ ${\mathrm{H}}^{+}$

$\mathrm{pH}=-\log {10}^{-2}$

$\mathrm{pH}=2$

$$\begin{gathered} \mathrm{pOH}=14-2 \\ \mathrm{pOH}=12 \end{gathered}$$

Step 2 - Calculation of pH of $0.01\mathrm{M}$ NaOH -

$0.01\mathrm{M}\mathrm{NaOH}$ contains ${10}^{-2}\mathrm{M}{\mathrm{OH}}^{-}$

$\mathrm{pOH}=-\log {10}^{-2}$

$\mathrm{pOH}=2$

Step 3 - Calculation of pH of $0.01\mathrm{M}$ ${\mathrm{CH}}_3\mathrm{COONa}$ -

${\mathrm{CH}}_3\mathrm{COONa}$ is a salt of weak acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$ and strong base $\left(\mathrm{NaOH}\right)$.

So, its $\mathrm{pH}$ will be greater than $7$ and $\mathrm{pOH}$ will be less than $7$.

$\mathrm{pOH}<7$ (but not less than 2 as it is not a strong acid but a salt of a strong acid).

Step 4 - Calculation of pH of $0.01\mathrm{M}$ $\mathrm{NaCl}$ -

$\mathrm{NaCl}$ is a salt of strong base $\left(\mathrm{NaOH}\right)$ and strong acid $\left(\mathrm{HCl}\right)$.

So, $\mathrm{pH}=7$

$\mathrm{pOH}=7$

So, the order of the following solutions in the decreasing order of $\mathrm{pOH}=\left(\mathrm{A}\right)>\left(\mathrm{D}\right)>\left(\mathrm{C}\right)>\left(\mathrm{B}\right)$

Hence, option D is the correct answer.