WAEC 2016 Chemistry · Question 41

How many moles of oxygen would contain 1.204 x 1024 molecules? [Avogadro's constant (NA) = 6.02 x 1023]

Options

Answer: B. 2

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We're asked to convert a number of molecules into moles using Avogadro's constant. Step 2: The concept One mole of any substance contains 6.02 × 10²³ particles (Avogadro's number, N_A). Number of moles = Number of molecules ÷ Avogadro's constant Step 3: Solution n = (1.204 × 10²⁴) ÷ (6.02 × 10²³) Divide the numbers: 1.204 ÷ 6.02 = 0.2 Divide the powers of ten: 10²⁴ ÷ 10²³ = 10¹ n = 0.2 × 10 = 2 moles Check: 2 × 6.02 × 10²³ = 12.04 × 10²³ = 1.204 × 10²⁴ ✓ Step 4: Answer The correct option is B. 2. Tip: To avoid slips with powers of ten, rewrite the large number so it has the same power as N_A. Here 1.204 × 10²⁴ = 12.04 × 10²³, so 12.04 ÷ 6.02 = 2 straight away. Also, the question asks for moles of oxygen molecules (O₂), so no extra conversion to atoms is needed.

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