$A$ solution which is resistant to change of $pH$ upon the addition of an acid or a base is known as

Calculate the $pH$ of the solution in which $0.2 \, M \, NH_{4}Cl$ and $0.1 \, M \, NH_{3}$ are present. The $pK_{b}$ of ammonia solution is $4.75$.

At $298 \text{ K}$, a certain buffer solution contains equal concentrations of $X^-$ and $HX$. The $K_b$ for $X^-$ is $10^{-10}$. What is the $pH$ of this buffer solution?

Calculate the $pH$ of a buffer solution containing $0.04 \ M$ $NaF$ and $0.02 \ M$ $HF$ $[pK_a = 3.142]$.

$20 \ mL$ 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 $NH_4OH = 4.7$].

If the $pH$ of a mixture of $10 \, mL$ of $0.1 \, M \, NH_4OH$ and $10 \, mL$ of $1 \, M \, NH_4Cl$ solution is $8$,the $pK_b$ value of $NH_4OH$ is then closest to