Given below are two statements: one is labelled as Assertion $\mathrm{A}$ and the other is labelled as Reason $\mathrm{R}$.

Assertion $\mathrm{A}$ :Phenolphthalein is a $\mathrm{pH}$ dependent indicator, remains colourless in acidic solution and gives pink colour in basic medium

Reason $\mathrm{R}$ : Phenolphthalein is a weak acid. It doesn't dissociate in basic medium.

In the light of the above statements, choose the most appropriate answer from the options given below

At $600 \mathrm{K},2 \mathrm{mol}$ of $\mathrm{NO}$ are mixed with $1 \mathrm{mol}$ of ${\mathrm{O}}_2$.

$2{\mathrm{NO}}_{\left(\mathrm{g}\right)}+{\mathrm{O}}_2\left(\mathrm{g}\right)\rightleftarrows 2{\mathrm{NO}}_2\left(\mathrm{g}\right)$

The reaction occurring as above comes to equilibrium under a total pressure of $1$ atm. Analysis of the system shows that $0.6 \mathrm{mol}$ of oxygen are present at equilibrium. The equilibrium constant ${\mathrm{K}}_{\mathrm{p}}$ for the reaction is____. Give an answer to the nearest integer value.

A litre of buffer solution contains $0.1$ mole of each of ${\mathrm{NH}}_3$ and ${\mathrm{NH}}_4\mathrm{Cl}$. On the addition of $0.02$ mole of $\mathrm{HCl}$ by dissolving gaseous $\mathrm{HCl}$, the $\mathrm{pH}$ of the solution is found to be $\_\_\_\_\_\times {10}^{-3}$ (Nearest integer)

Given: ${\mathrm{pK}}_{\mathrm{b}}\left({\mathrm{NH}}_3\right)=4.745$

$\log 2=0.301$

$\log 3=0.477$

$\mathrm{T}=298 \mathrm{K}]$