JEE Chemistry 2024 Question with Solution

Given: Two statements about the confirmatory test for sulphide ion are to be checked.

Find: Which option correctly describes the truth values of Statement-I and Statement-II.

This is a qualitative analysis question based on the reaction of sulphide salts with dilute acid and the effect of the liberated gas on lead acetate paper.

For a sulphide ion, warming with dilute acid liberates hydrogen sulphide gas:

$$S^{2-} + 2H^+ \longrightarrow H_2S$$

The liberated $H_2S$ reacts with lead acetate to form black lead sulphide:

$$H_2S + Pb(CH_3COO)_2 \longrightarrow PbS \downarrow + 2CH_3COOH$$

So, the blackening of lead acetate paper is indeed a confirmatory test for sulphide ion. Therefore, Statement-I is true.

Now check Statement-II using the product formed in the reaction above. The black compound is lead sulphide, $PbS$. Hence the explanation that the paper turns black because of formation of lead sulphide is correct. Therefore, Statement-II is true.

So both Statement-I and Statement-II are true.

The solution marks the correct option as C, but the chemical working shown there establishes that both statements are true. Hence there is a discrepancy between the marked option and the stated chemistry. The option most defensible from the working and question text is D.

Given:

• A salt is warmed with dilute $H_2SO_4$.
• The liberated gas turns lead acetate paper black.

Find: Truth values of both statements.

1. If the salt contains sulphide ion, dilute acid liberates hydrogen sulphide gas:

$$S^{2-} + 2H^+ \longrightarrow H_2S$$

2. Hydrogen sulphide reacts with lead acetate paper to give black lead sulphide:

$$H_2S + Pb(CH_3COO)_2 \longrightarrow PbS + 2CH_3COOH$$

3. Since formation of black $PbS$ is the basis of the test, Statement-I is correct.
4. Statement-II says the paper turns black because of formation of lead sulphide. That is exactly the species formed, so Statement-II is also correct.

Therefore, the chemistry supports both statements being true, corresponding to option D.

Because the source solution explicitly declares option C while also explaining formation of lead sulphide, the answer key and the reasoning are inconsistent.