Question

Given below are two statements:
Statement-I: The gas liberated on warming a salt with dil H₂SO₄, turns a piece of paper dipped in lead acetate into black, it is a confirmatory test for sulphide ion.
Statement-II: In statement-I the colour of paper turns black because of formation of lead sulphite. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement-I and Statement-II are false
• B. Statement-I is false but Statement-II is true
• C. Statement-I is true but Statement-II is false
• D. Both Statement-I and Statement-II are true.

Answer 1

The Correct Option is C

This query requires evaluating two assertions concerning a chemical assay for the sulfide ion and selecting the most fitting description of their accuracy.

Principles Utilized:

The fundamental concepts underpinning this problem relate to qualitative inorganic analysis, specifically the confirmatory assay for the sulfide anion (\(S^{2-}\)). The essential chemical transformations are:

1. The interaction between a sulfide salt and a dilute acid, yielding hydrogen sulfide (\(H_2S\)) gas.
2. The reaction of hydrogen sulfide gas with a lead(II) acetate solution, resulting in a precipitate.

Familiarity with the chemical formulas and physical characteristics (particularly color) of the resulting compounds, such as lead sulfide (\(PbS\)) and lead sulfite (\(PbSO_3\)), is necessary.

Solution Breakdown:

Stage 1: Assessment of Assertion-I.

Assertion-I delineates a procedural methodology and its outcome. Examination is as follows:

• Reaction 1: Heating a salt with dilute H₂SO₄. If the salt contains the sulfide ion (\(S^{2-}\)), it will react with the acid to generate hydrogen sulfide gas, which possesses a distinctive odor of rotten eggs. \[ S^{2-}(aq) + 2H^+(aq) \longrightarrow H_2S(g) \]
• Reaction 2: The evolved gas causes a lead acetate-dipped paper to turn black. The gas, \(H_2S\), interacts with lead(II) acetate (\(Pb(CH_3COO)_2\)) solution. \[ H_2S(g) + Pb(CH_3COO)_2(aq) \longrightarrow PbS(s) \downarrow + 2CH_3COOH(aq) \]
• Observation and Determination: The product of this reaction is lead(II) sulfide (\(PbS\)), a black solid precipitate. The formation of this characteristic black precipitate serves as a recognized and dependable confirmatory test for the presence of the sulfide ion.

Consequently, Assertion-I accurately represents the confirmatory test for sulfide ions.

Determination for Assertion-I: Assertion-I is factual.

Stage 2: Assessment of Assertion-II.

Assertion-II proposes a rationale for the observation recorded in Assertion-I, attributing the blackening of the paper to the formation of lead sulfite (\(PbSO_3\)).

• As established in Stage 1, the black precipitate generated is lead sulfide, identified by the chemical formula \(PbS\).
• Lead sulfite, with the chemical formula \(PbSO_3\), is a white solid. Its formation results from the reaction between a sulfite ion (\(SO_3^{2-}\)) and a soluble lead salt.
• The chemical entity cited in Assertion-II, lead sulfite, is erroneous.

Therefore, Assertion-II offers an incorrect explanation for the observed phenomenon.

Determination for Assertion-II: Assertion-II is unsubstantiated.

Final Calculation & Outcome:

Based on the preceding analysis:

• Assertion-I is valid.
• Assertion-II is invalid.

Accordingly, the most precise conclusion is that Assertion-I is true and Assertion-II is false.