On combining which of the following anions with $NH_4^{\oplus}$ will the $pH$ of the resulting salt solution be independent of the concentration?

If $pK_b$ for $CN^{-}$ at $25\,^{\circ}C$ is $3.7$,what is the $pH$ of a $0.5\, M$ aqueous $NaCN$ solution?

$A$ solution of sodium borate has a $pH$ of approximately

The aqueous solution of disodium hydrogen phosphate $(Na_2HPO_4)$ is:

What is the $pH$ of a $K_2S$ solution?

Explain the following with reasons:
$(i)$ $NaCl$ solution is neutral. The solution of strong acid and strong base salts is neutral.
$(ii)$ $NaCH_3COO$ solution is basic. The solution of strong base and weak acid salt is basic.
$(iii)$ $NH_4Cl$ solution is acidic. The solution of weak base and strong acid salt is acidic.
$(iv)$ The solution of $NH_4CH_3COO$ is (almost) neutral. The solution of weak acid - weak base salts is almost neutral.