The manufacture of sulphuric acid involves the following steps. Ident…

The correct answer is Option (1) → X₁ = S, X₂ = V₂O₅, X₃ = SO₃, X₄ = H₂S₂O₇

Sulphuric acid is made by the Contact process in three steps. First, sulphur is burnt in air to give \( \ce{SO2} \), so X₁ = S:

    \( \displaystyle \ce{S(s) + O2(g) -> SO2(g)} \)

Then \( \ce{SO2} \) is oxidised by oxygen over the catalyst vanadium pentoxide, so X₂ = V₂O₅ and X₃ = SO₃:

    \( \displaystyle \ce{2SO2(g) + O2(g)} \xrightarrow{\ce{V2O5}} \ce{2SO3(g)} \)

Last, \( \ce{SO3} \) is absorbed in sulphuric acid to give oleum, so X₄ = H₂S₂O₇. Oleum diluted with water gives sulphuric acid of the desired strength:

    \( \displaystyle \ce{SO3 + H2SO4 -> H2S2O7} \)

    \( \displaystyle \ce{H2S2O7 + H2O -> 2H2SO4} \)

• Option (2) X₁ = SO₃, X₂ = V₂O₃, X₃ = S, X₄ = H₂SO₅ — Incorrect. \( \ce{SO3} \) cannot burn in oxygen to give \( \ce{SO2} \), the catalyst is \( \ce{V2O5} \) and not \( \ce{V2O3} \), and \( \ce{SO2} \) is oxidised to \( \ce{SO3} \), not to S.

• Option (3) X₁ = H₂S, X₂ = VO₂, X₃ = SO₃, X₄ = H₂SO₅ — Incorrect. The catalyst is \( \ce{V2O5} \), not \( \ce{VO2} \). \( \ce{SO3} \) with \( \ce{H2SO4} \) gives oleum, \( \ce{H2S2O7} \), not \( \ce{H2SO5} \) (peroxomonosulphuric acid, which has an O–O bond).

• Option (4) X₁ = S, X₂ = V₂O₃, X₃ = SO₃, X₄ = H₂S₂O₇ — Incorrect. X₁, X₃ and X₄ are right, but the catalyst is \( \ce{V2O5} \) (vanadium +5), not \( \ce{V2O3} \) (vanadium +3).

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Sulphuric Acid – “… Sulphuric acid is manufactured by the Contact Process which involves three steps:
(i) burning of sulphur or sulphide ores in air to generate SO₂.
(ii) conversion of SO₂ to SO₃ by the reaction with oxygen in the presence of a catalyst (V₂O₅), and
(iii) absorption of SO₃ in H₂SO₄ to give Oleum (H₂S₂O₇).”