SSC CGL 2024 Tier-I · 12 September 2024, Shift 1 · General Awareness · Question 18 of 99

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 6 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Calculate the oxidation number of ‘S’ in H 2 S 2 O 7 .

(a)3
(b)7
(c)2
(d)6

Answer

Why

Correct — D. Call the oxidation number of sulphur x and make the neutral molecule add up to zero.

H is +1, O is −2
2(+1) + 2x + 7(−2) = 0
2 + 2x − 14 = 0

2x = 12
x = +6 → option (d)

The two sulphur atoms in H₂S₂O₇ sit in equivalent positions, so each one carries +6 — the 12 is the pair's total, not one atom's value.

Why the others are wrong

(a)
3 — 3 comes from taking the +6 as something shared between the two sulphur atoms and halving it again. The x in the balance already stands for one atom.
(b)
7 — 7 is the number of oxygen atoms in the formula, not an oxidation number. Sulphur has six valence electrons, so +6 is the highest state available to it.
(c)
2 — 2 ignores how much oxygen the molecule carries. Seven oxygens at −2 make −14, and only +6 on each sulphur, with +2 from the hydrogens, balances that.

Concept

An oxidation number is assigned by convention, not measured. Fix the atoms whose values are settled and solve for the one left.

In an ordinary compound hydrogen is +1 and oxygen is −2, and a neutral molecule sums to zero. For an ion the sum equals the charge instead.

H₂S₂O₇ is pyrosulphuric acid, also called disulphuric acid, the species that gives oleum its character when sulphur trioxide is dissolved in sulphuric acid. Its sulphur is +6, the same value sulphur takes in H₂SO₄.

Two sulphur atoms do not mean two different answers. Where the atoms are in the same chemical environment, the balance gives one value that applies to each.

Key facts

In a neutral compound the oxidation numbers of all atoms add up to zero.
Hydrogen is normally +1 and oxygen normally −2 when oxidation numbers are assigned.
Sulphur is +6 in H₂S₂O₇ and also +6 in H₂SO₄.
H₂S₂O₇ is pyrosulphuric acid, the acid present in oleum.

Study next

Assigning oxidation numbers in ions, where the total equals the ion's chargeOxidation states of sulphur across H₂S, SO₂, H₂SO₃ and H₂SO₄Peroxides and metal hydrides, the cases where the −2 and +1 conventions break

Common traps

Reading the 7 in the formula as an oxidation number is the fastest wrong answer here.
Solving for both sulphur atoms together gives 12, and halving that a second time gives 3.

SSC's chemistry slot asks for a formula, a product or an oxidation state — sulphuric acid returns as the zinc reaction on 12 Sep 2024, 12:30, GA Q.19 and again on 23 Sep 2024, 12:30, GA Q.9.

Related PYQs

No directly related past PYQ was found.