Question

$100\mathrm{mL}$ of $0.1\mathrm{M}\mathrm{HCl}$ is taken in a beaker and to it, $100\mathrm{mL}$ of $0.1\mathrm{M}\mathrm{NaOH}$ is added in steps of $2\mathrm{mL}$ and the $\mathrm{pH}$ is continuously measured. Which of the following graphs correctly depicts the change in $\mathrm{pH}$?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Explanation for the correct option:

C)

Step 1: Given data:

Volume of $\mathrm{HCl}$, ${\mathrm{V}}_1=100\mathrm{mL}$

Volume of $\mathrm{NaOH}$, ${\mathrm{V}}_2=100\mathrm{mL}$

Molarity of $\mathrm{HCl}$, ${\mathrm{M}}_1=0.1\mathrm{M}$

Molarity of $\mathrm{NaOH}$, ${\mathrm{M}}_2=0.1\mathrm{M}$

Step 2: Plot the graph obtained initially when only $\mathbf{HCl}$ is present:

1. Initially, in the beaker, only $\mathrm{HCl}$is present.
2. $\therefore$The $\mathrm{pH}<7\left(\because \mathrm{Acidic}\mathrm{solution}\right)$

Step 3: Plot the graph obtained when $\mathbf{HCl}$is titrated against $\mathbf{NaOH}$:

1. When $2\mathrm{mL}$ of $\mathrm{NaOH}$ is added continuously, the $\mathrm{pH}$ increases.
2. As the concentration of both $\mathrm{HCl}$and $\mathrm{NaOH}$ is the same, an equivalent point can be observed.
3. At equivalent point, both $\mathrm{HCl}$and $\mathrm{NaOH}$ are in equal volumes.
4. $\therefore$The $\mathrm{pH}=7\left(\because \mathrm{Neutral}\mathrm{solution}\right)$
5. The graph at this point is as follows;

Step 4: Plot the graph obtained after reaching the equivalence point:

1. If $\mathrm{NaOH}$ is added even after the equivalence point, the $\mathrm{pH}$ value still increases and the graph obtained is as follows;
2. 

Explanation for the incorrect options:

A)

1. When equal concentrations of strong acid and strong base are present, the graph at first remains constant, then increases and again remains constant after reaching some point.
2. But here the graph shown is increasing from the initial point which is incorrect.

B)

When strong acid and strong base which are having equal concentrations are added, the $\mathrm{pH}=7\left(\because \mathrm{Neutral}\mathrm{solution}\right)$ will be obtained at the equivalence point only.

D)

1. According to the given data, initially in the beaker $\mathrm{HCl}$is present and then $\mathrm{NaOH}$ is added continuously.
2. But in this graph, it is showing that initially the volume of $\mathrm{NaOH}$ is more and volume of $\mathrm{HCl}$ is increasing gradually, which is incorrect.

Hence, option C is the correct answer.