$\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{A}^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{O}+\mathrm{HA}$
$\mathrm{OH^-}+\mathrm{HA} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{A}^{-}$
If acid is added, it reacts with $A^-$ to form undissociated $HA$. Similarly, if base/alkali is added, $OH^-$ combines with $HA$ to give $H_2O$ and Aand thus, maintains the acidity/alkalinity of buffer solution.
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${\text{2PQ}} \rightleftharpoons {{\text{P}}_2}{\text{ + }}{{\text{Q}}_2}\,;\,\,\,\,{{\text{K}}_1}{\text{ = 2}}{\text{.5 }} \times \,{\text{1}}{{\text{0}}^5}\,$
${\text{PQ + }}\frac{1}{2}{R_2} \rightleftharpoons PQR\,;\,\,\,\,{{\text{K}}_2}{\text{ = 5 }} \times \,{\text{1}}{{\text{0}}^{ - 3}}\,$
The value of equilibrium constant for reaction
$\frac{1}{2}{{\text{P}}_2}{\text{ + }}\frac{1}{2}{{\text{Q}}_2}{\text{ + }}\frac{1}{2}{R_2} \rightleftharpoons PQR\,$ is
