A compound that could be dried by using conc. tetraoxosulphate (VI) acid and not by calcium oxide is likely to be

Analyze the question: We need a compound (usually a gas) that can be dried with concentrated H₂SO₄ (an acidic drying agent) but not with CaO (a basic drying agent). So the compound must react with CaO, which means it is acidic, and it must not react with H₂SO₄.

Concept:

• Conc. H₂SO₄ is an acidic drying agent. It dries neutral and acidic gases (e.g. CO₂, SO₂, HCl, Cl₂) but not basic gases like NH₃, which it would react with.
• CaO is a basic drying agent. It dries neutral and basic gases (e.g. NH₃) but not acidic gases, because it reacts with them: CaO + CO₂ → CaCO₃.

Going through the options:

• A. Deliquescent salt: This absorbs water from the air and is not a gas to be dried in this sense. It does not fit the pattern.
• B. Neutral salt: It would react with neither agent, so both could dry it. It does not fit.
• C. Alkaline gas (e.g. NH₃): It reacts with conc. H₂SO₄, so it is dried by CaO and not by H₂SO₄. This is the opposite of what the question asks.
• D. Acid anhydride: Acid anhydrides are acidic oxides such as CO₂, SO₂ and SO₃ (with SO₃ being a special case, since it dissolves in conc. H₂SO₄). They react with basic CaO, so CaO cannot be used. Most, like CO₂ and SO₂, do not react with conc. H₂SO₄, so it can dry them. This fits.

Tip: Match opposites. Acidic gases need acidic drying agents (conc. H₂SO₄), and basic gases need basic drying agents (CaO). Remember that NH₃ is the classic gas dried by CaO only.