1. Tardigrade
  2. Question
  3. Chemistry
  4. A solution of weak acid HA is being titrated with 0.1 M NaOH solution. End point is reached on addition of x mL of NaOH. To the above solution, (x/2) mL of 0.10 M HCl is added further. pH of the resulting solution was found to be 5.3. Determine pH at the end point if initial concentration of weak acid was 0.10 M.

Question Error Report

Thank you for reporting, we will resolve it shortly

Back to Question

Q. A solution of weak acid $HA$ is being titrated with $0.1\, M$ $NaOH$ solution. End point is reached on addition of $x \,mL$ of $NaOH$. To the above solution, $\frac{x}{2} mL$ of $0.10\, M$ $HCl$ is added further. $pH$ of the resulting solution was found to be $5.3$. Determine $pH$ at the end point if initial concentration of weak acid was $0.10\, M$.

Equilibrium

Solution:

image
$pH = pK _{ a }+\log \frac{\left[ A ^{-}\right]}{[ HA ]}= pK _{ a }=5.3$
$ \Rightarrow K _{ a }=5 \times 10^{-6}$
$A ^{-}+ H _{2} O \rightleftharpoons HA + OH ^{-}$
$K _{ a }= K _{ a }=\frac{ k _{ w }}{ k _{ a }}=2 \times 10^{-9}$
$\Rightarrow\left[ OH ^{-}\right]=\sqrt{ K _{ h } C }$
$=\sqrt{2 \times 10^{-9} \times 0.05}=10^{-8}$
$\Rightarrow pOH =5$ and $pH =9$