Question

Which of the following plot represents the graph of pH against volume of alkali added in the titration of $NaOH$ and $HCl$
A.
B.
C.
D.

Answer 1

We need to know that $pH$ is mainly used to find out the solution which is acidic, basic or neutral. And the range of $pH$ from one to $14$ . The solution is neutral if the $pH$ is $7$ . And if it is less than $7$ means, the solution is acidic and $pH$ is greater than seven means, the solution is basic. And $pH$ mainly measures the relative amount of free hydrogen which is present in the water.

Complete answer:
While adding alkali to acid, there occurs a change in $pH$. And the acidity of acid becomes decreasing and the basicity will start to increase. But there is no sudden increase in the $pH$ Hence, option (A) is incorrect.
Here, the base is added to the acid solution. Hence, $pH$of the solution is low at the initial stage. But, in this graph, the plot of $pH$ starts from its higher range. Hence, option (B) is correct.
When sodium hydroxide is added to hydrochloric acid, $pH$ of the solution remains the same at the initial stage. After that it suddenly increases $pH$. Then it became almost the same. After that there is no change in the $pH$. Therefore, the graph of pH against volume of alkali added in the titration of $NaOH$ and $HCl$ is,

Hence, option (C) is correct.
This is not the correct graphical representation of pH against volume. Hence, the option (D) is incorrect.

Hence, option (C) is correct.

Note:
We need to know that when the alkali and acid are mixing in the right amounts, the reaction is wound up with the formation of a neutral solution. If water or base is added to an acid, the pH of the acid will be increasing. Because, there is a decrease in the acidity and the acid becomes less acidic. If the water is added to an alkali solution, the pH of the base becomes near $7$ due to the decrease in the concentration of hydroxide ions.