The number of sulphur atoms in 3.20g of SO$_{2(g)}$ is [O = 16.0;S = 32.0; Avogadro constant = 6.02 x 10$^{23}$]

Analyze the question: We need the number of sulphur atoms in 3.20 g of SO₂. Each SO₂ molecule contains one S atom, so the number of S atoms equals the number of SO₂ molecules.

Concept:

• Moles = mass ÷ molar mass
• Number of particles = moles × Avogadro constant (6.02 × 10²³)

Solution:

1. Molar mass of SO₂ = 32.0 + (2 × 16.0) = 64.0 g/mol

2. Moles of SO₂ = 3.20 ÷ 64.0 = 0.05 mol

3. Number of molecules = 0.05 × 6.02 × 10²³ = 3.01 × 10²² molecules

4. Each molecule has 1 sulphur atom, so the number of S atoms = 3.01 × 10²²

Answer: Option A (3.01 × 10²²).

Tip: Always check the subscript of the element you are counting. Here S has no subscript, so it is 1 atom per molecule. If the question had asked for oxygen atoms, you would multiply by 2 (giving 6.02 × 10²²). That is exactly what option B is designed to trap you with, so read which atom is being asked about.