Identify reagent 'A' used in the following reaction?

Acidic buffer solution is prepared by mixing proportionate quantity of

Which among the following is NOT a feature of $\mathrm{S}_{\mathrm{N}} 2$ mechanism?

3.4 moles of an ideal gas occupies volume of 68 mL at 300 K . What would be the pressure of gas? $\left(\mathrm{R}=8.314 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}\right)$