WAEC 2021 Chemistry · Question 5
How many molecules of oxygen would occupy a volume of 2.24 cm at s.t.p? [Molar volume at s.t.p = 22,400 cm, Avogadro's number = 6.02 x 10]
Options
- A.3.01 x 10
- B.3.01 x 10
- C.6.02 x 10
- D.6.02 x 10
Answer: C. 6.02 x 10
Explanation written with AI and checked against the answer key
Analyze the question: We need the number of O₂ molecules in 2.24 cm³ of gas at s.t.p. Concept: At s.t.p., 1 mole of any gas occupies 22,400 cm³ and contains 6.02 × 10²³ molecules (Avogadro's number). Solution: Number of moles = volume ÷ molar volume n = 2.24 ÷ 22,400 = 1 × 10⁻⁴ mol Number of molecules = n × Avogadro's number = 1 × 10⁻⁴ × 6.02 × 10²³ = 6.02 × 10¹⁹ molecules Tip: Look at the ratio of the given volume to the molar volume first. Here, 2.24 is 1/10,000 of 22,400, so the answer is simply Avogadro's number divided by 10⁴, which gives 10²³ ÷ 10⁴ = 10¹⁹. Always check that the units (cm³ with cm³) match before you divide.AI can make mistakes. Report anything that looks wrong.
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