Which of the following mixtures act$(s)$ as a buffer solution?

$A$ buffer solution contains $1 \ mol$ of ammonium sulphate and $1 \ mol$ of $NH_4OH$ $(K_b = 10^{-5})$. The $pH$ of the solution will be:

If the $pH$ of a buffer solution containing $0.1 \ M$ of monoacidic base and $0.01 \ M$ of its salt is $10.5$,the $pK_a$ of the conjugate acid is: (in $.5$)

$20 \ mL$ of $0.1 \ M \ H_2SO_4$ is added to $30 \ mL$ of $0.2 \ M \ NH_4OH$ solution. The $pH$ of the resultant mixture is [$pK_b$ of $NH_4OH = 4.7$].

$A$ student needs to prepare a buffer solution of propanoic acid and its sodium salt with $pH$ $4$. The ratio of $\frac{[CH_{3}CH_{2}COO^{-}]}{[CH_{3}CH_{2}COOH]}$ required to make the buffer is .....
Given: $K_{a}(CH_{3}CH_{2}COOH) = 1.3 \times 10^{-5}$

The acidic buffer $(HX + NaX)$ can be best used to maintain the $pH$ in the range $(4 - 6)$. Find the $pK_b$ of $X^-$.