For a weak acid $(HA)$,$pK_a = 4.5$. The $pOH$ of a buffer solution of $HA$ in which $50\%$ of the acid is ionized is:

$3 \; g$ 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.

During the titration of $N/10$ acetic acid with $N/10 \, NaOH$,what is the $pH$ of the solution when $25\%$,$50\%$,and $75\%$ of the titration is complete? $[K_a = 10^{-5}]$

Calculate the molar ratio of a weak acid $HA$ $(K_a=10^{-6})$ and its salt with a strong base,so that the $pH$ of the buffer solution is $6$.

Assertion : Buffer system of carbonic acid and sodium bicarbonate is used for the precipitation of hydroxides of third group elements.
Reason : It maintains the $pH$ to a constant value,about $7.4.$

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