When an acid or alkali is mixed with a buffer solution,the $pH$ of the buffer solution:

The $OH^{-}$ concentration in a mixture of $5.0 \ mL$ of $0.0504 \ M \ NH_{4}Cl$ and $2 \ mL$ of $0.0210 \ M \ NH_{3}$ solution is $x \times 10^{-6} \ M$. The value of $x$ is ..... .
(Nearest integer)
$[ \text{Given } K_{w}=1 \times 10^{-14} \text{ and } K_{b}=1.8 \times 10^{-5} ]$

Which of the following mixtures will form a buffer solution?

What is $[H^{+}]$ in $mol/L$ of a solution that is $0.20\, M$ in $CH_3COONa$ and $0.10\, M$ in $CH_3COOH$? $K_a$ for $CH_3COOH = 1.8 \times 10^{-5}$.

$0.1 \ M, 60 \ mL$ $NH_4Cl$ $(K_h = 10^{-9})$ is titrated against $0.2 \ M$ $NaOH$. What is the $pH$ at the $\frac{1}{3}$ equivalence point?

$A$ buffer solution is prepared by mixing $10 \ mL$ of $1.0 \ M$ acetic acid and $20 \ mL$ of $0.5 \ M$ sodium acetate and then diluted to $100 \ mL$ with distilled water. The $pH$ of the buffer solution is ($pK_a$ of acetic acid is $4.76$).