What is the percentage by mass of H in  CH$_3$COOH ?

Analyze the question: We need the percentage by mass of hydrogen in ethanoic acid (acetic acid), CH₃COOH.

Concept: Percentage by mass of an element = (total mass of that element in one mole ÷ molar mass of compound) × 100

Solution:

1. Count the atoms in CH₃COOH (C₂H₄O₂): • C: 2 • H: 3 + 1 = 4 • O: 2

2. Calculate the molar mass (C = 12, H = 1, O = 16): • C: 2 × 12 = 24 • H: 4 × 1 = 4 • O: 2 × 16 = 32 • Total = 60 g/mol

3. Calculate the percentage of H: • % H = (4 ÷ 60) × 100 = 6.67% ≈ 6.7%

Answer: The value 6.

Tip: Always write the molecular formula first (CH₃COOH = C₂H₄O₂) so you don't miss the hydrogen in the COOH group. Many students count only the 3 H in CH₃ and get 5.0% (option B), which is the trap here.