If the molecular mass of tetraoxosulphate (VI) acid is 98, calculate its vapour density.

Analyze the question You're given the molecular mass of H₂SO₄ (tetraoxosulphate (VI) acid) as 98 and asked for its vapour density.

Concept Vapour density (VD) compares the mass of a gas or vapour to the mass of an equal volume of hydrogen. The relationship is:

Molecular mass = 2 × Vapour density

so

Vapour density = Molecular mass ÷ 2

This works because hydrogen exists as H₂, which has a molecular mass of 2.

Solution Vapour density = 98 ÷ 2 = 49

Checking the options:

• A (196) is 98 × 2, which is the wrong direction.
• B (49) is 98 ÷ 2, which is correct.
• C (106) and D (82) don't come from any valid relationship.

Answer: B (49)

Exam tip Remember "VD is half of the molecular mass", i.e. M = 2 × VD. If you're given VD and asked for molecular mass, multiply by 2. If you're given molecular mass and asked for VD, divide by 2. Many students lose marks by multiplying when they should divide, so check which direction you're going.