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

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 $4.75$.

Class $XII$ students were asked to prepare one litre of buffer solution of $pH$ $8.26$ by their chemistry teacher. The amount of ammonium chloride to be dissolved by the student in $0.2 \, M$ ammonia solution to make one litre of the buffer is $..... \, g$ (Given $pK_b(NH_3)=4.74$; Molar mass of $NH_3=17 \, g \, mol^{-1}$; Molar mass of $NH_4Cl =53.5 \, g \, mol^{-1}$)

Find $pH$ of the resultant solution formed by the addition of $500 \ mL$ $0.1 \ M$ $Ba(OH)_2$ and $500 \ mL$ $0.6 \ M$ $NH_4Cl$ in $1 \ L$ pure $H_2O$ at $25 \ ^oC$ (Use $\log \ 2 = 0.3$,$\log \ 3 = 0.5$,$\log \ 5 = 0.7$,$K_b(NH_3) = 10^{-5}$)

$3 \; g$ of acetic acid is added to $250 \; mL$ of $0.1 \; M \; HCl$ and the solution is made up to $500 \; mL$. To $20 \; mL$ of this solution,$\frac{1}{2} \; mL$ of $5 \; M \; NaOH$ is added. The $pH$ of the solution is: [Given: $pK_{a}$ of acetic acid $= 4.75$,molar mass of acetic acid $= 60 \; g/mol$,$\log 3 = 0.4771$]. Neglect any changes in volume.

Find the $pH$ when $0.2 \, mol$ of $HCl$ is added to $1 \, L$ of a solution containing $1 \, M$ $CH_3COOH$ and $1 \, M$ $CH_3COO^-$. Assume the total volume remains $1 \, L$. Given $K_a$ for $CH_3COOH = 1.8 \times 10^{-5}$.