Take C = 12, H = 1, O = 16. What is the percentage by mass of oxygen in ethanoic acid, CH₃COOH?
Correct answer A. 53.3%
Explanation
Molar mass of C₂H₄O₂ = 24 + 4 + 32 = 60 g/mol. Percentage of oxygen = (32/60) × 100 = 53.3%. The value 40.0% is the carbon percentage, and 26.7% counts only one oxygen atom.
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