What is the $pH$ of a solution prepared by mixing $20 \ mL$ of $0.250 \ M$ sodium formate with $30 \ mL$ of $0.100 \ M$ $HCl$ ($pK_a$ for $HCOOH = 3.75$):-

Which of the following does not form a buffer solution?

The $pH$ of a buffer solution containing $0.2 \ mol/L$ $CH_3COONa$ and $1.5 \ mol/L$ $CH_3COOH$ is ($K_a$ for acetic acid is $1.8 \times 10^{-5}$).

Find the value of $\frac{[NaCN]}{[HCN]}$ given the $pH$ of the buffer solution of $NaCN$ and $HCN$ is $9.3$ and $K_a$ of $HCN$ is $5 \times 10^{-10}$.

Buffer Solution	Volume of $0.1 \ M$ Weak Acid (mL)	Volume of $0.1 \ M$ Sodium Salt (mL)
$I$	$4.0$	$4.0$
$II$	$4.0$	$40.0$
$III$	$40.0$	$4.0$
$IV$	$0.1$	$10.0$

Which of the two sets of buffer solutions have the least $pH$?

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)
$[ \text{Given } K_{w}=1 \times 10^{-14} \text{ and } K_{b}=1.8 \times 10^{-5} ]$