Which from following species acts as base ${ }^1$, according to Bronsted-Lowry theory?

$$\mathrm{HCl}+\mathrm{NH}_3 \text { 目的 } \mathrm{NH}_4^{+}+\mathrm{Cl}^{-} $$

What is the ratio of concentration of salt to concentration of weak acid in buffer solution to maintain its pH value $7.2\left(\mathrm{pK}_{\mathrm{a}}=6.2\right)$.

What is the value of pOH if a buffer solution is prepared by mixing equal volumes of $0.4 \mathrm{~M} \mathrm{~NH}_4 \mathrm{OH}$ and $0.5 \mathrm{~M} \mathrm{~NH}_4 \mathrm{Cl}$ solutions. $\left(\mathrm{pK}_{\mathrm{b}}=4.730\right)$

Which of the following salt solutions turns red litmus blue?