JEE Chemistry 2023 Question with Solution

Given: $K_{\text{phenolphthalein}} = 4 \times 10^{-10}$.

Find: Which statements about phenolphthalein are correct, and hence identify the correct option.

The solution uses the indicator constant to find the indicator range:

$$\text{p}K_n = -\log(4 \times 10^{-10}) = 9.4$$

Therefore, the indicator range is:

$$\text{p}K_n \pm 1 \implies 8.4 \text{ to } 10.4$$

So, statement (B) is correct because phenolphthalein begins to change colour around $\text{pH} = 8.4$.

Statement (D) is also correct because phenolphthalein is colorless in acidic medium due to its unionized form.

Therefore, there are two correct statements, and the correct option is B.

Given: $K_{\text{phenolphthalein}} = 4 \times 10^{-10}$.

Find: Check each statement individually.

1. Calculate:

$$\text{p}K_n = -\log(4 \times 10^{-10}) = 9.4$$

2. Hence the colour-change interval is:

$$\text{p}K_n \pm 1 = 8.4 \text{ to } 10.4$$

3. From this:

• (B) is correct.
• (D) is correct.

Thus the number of correct statements is $2$. Therefore, the correct option is B.