Some Basic Concepts of Chemistry 2 Question 2
2. In an acid-base titration, $0.1 \mathrm{M} \mathrm{HCl}$ solution was added to the $\mathrm{NaOH}$ solution of unknown strength. Which of the following correctly shows the change of $\mathrm{pH}$ of the titration mixture in this experiment?
(2019 Main, 9 April II)
$(A)$
$(C)$
$(B)$
$(D)$
(b) $(A)$
(d) $(C)$ (c) $(B)$
(2019 Main, 9 April II)
Show Answer
Solution:
- The graph that shows the correct change of $\mathrm{pH}$ of the titration mixture in the experiment is
In this case, both the titrants are completely ionised.
$$ \mathrm{HCl}+\mathrm{NaOH} \rightleftharpoons \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O} $$
As $\stackrel{\oplus}{\mathrm{H}}$ is added to a basic solution, $\left[\mathrm{OH}^{\ominus}\right]$ decreases and $\left[\mathrm{H}^{+}\right]$ increases. Therefore, $\mathrm{pH}$ goes on decreasing. As the equivalence point is reached, $\left[\mathrm{OH}^{\ominus}\right]$ is rapidly reduced. After this point $\left[\mathrm{OH}^{\ominus}\right]$ decreases rapidly and $\mathrm{pH}$ of the solution remains fairly constant. Thus, there is an inflexion point at the equivalence point.
The difference in the volume of $\mathrm{NaOH}$ solution between the end point and the equivalence point is not significant for most of the commonly used indicators as there is a large change in the $\mathrm{pH}$ value around the equivalence point. Most of them change their colour across this $\mathrm{pH}$ change.
