Answer

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**Hint:**In this question, Henderson equation is used to calculate the value of $pOH$ which is further used to calculate the concentration of hydroxyl ions in the solution. Decrease in dissociation constant of an acid will increase the value of $pOH$ .

**Complete step by step answer:**

Here, it is given that the concentration of $N{{H}_{4}}OH$ is $0.2M$

The concentration of $N{{H}_{4}}Cl$ is $0.2M$

The concentration of $HCl$ is $0.001M$

The dissociation constant of a base is $2\times {{10}^{-5}}$

${{K}_{b}}=2\times {{10}^{-5}}$

$p{{K}_{b}}=-\log {{K}_{b}}$

Substituting the value in above formula we get,

$p{{K}_{b}}=-\log \left( 2\times {{10}^{-5}} \right)$

$p{{K}_{b}}=5-\log 2$

If we apply the Henderson equation ,

$pOH=p{{K}_{b}}+\log \left[ \dfrac{S}{B} \right]$

Where, $pOH$ is the basicity level

$p{{K}_{b}}$ is the dissociation constant of a base

$S$ is the concentration of salt

$B$ is the concentration of base

Now, if we substitute the values in the above formula we get,

$pOH=5-\log 2+\log \left[ \dfrac{0.2}{0.2} \right]$

$pOH=5-\log 2$

$\left[ O{{H}^{-}} \right]=2\times {{10}^{-5}}$

**Therefore, the correct option is A.**

**Additional information**

-$pH$ is defined as the scale to measure the acidity and basicity of an aqueous solution or liquid. The $pH$ before seven considers an acid and $pH$ after seven considers as a base.

-${{K}_{b}}$ is defined as the dissociation constant of a base. If the value of dissociation constant of a base is high then it is a strong base whereas if the value of the dissociation constant of base is low then it is a weak base.

Buffers consist of salt of conjugate base and acid. It is of two types:

A.Simple buffer- it is defined as a salt of weak acid and weak base.

B.Mixed buffer- it can be acidic and it can be basic.

**Note:**

The concentration of hydroxyl ions is calculated through the value of $pOH$ .If the value of $p{{K}_{b}}$ is lower then it is a weak base otherwise if the value of $p{{K}_{b}}$ is higher than it is a strong base.

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