The half-life of a first-order chemical reaction is 20 minutes. What percentage of the initial reactant concentration remains unreacted after 60 minutes?

Answer and explanation

Correct answer: B. 12.5%

  1. Step 1: Determine number of half-lives in 60 minutes: n = 60 / 20 = 3 half-lives. Step 2. For a first-order reaction: [A] = [A0] × (1/2)ⁿ. Step 3. After 3 half-lives: fraction remaining = (1/2)³ = 1/8 = 0.125 = 12.5%. Answer: 12.5%. Why not the others: 25.0% remains after 2 half-lives (40 min); 6.25% remains after 4 half-lives (80 min); 50.0% remains after 1 half-life (20 min).

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