Question Detail
Question
A certain first-order reaction A → B is 25% complete in 42 min at 25°C. What is the half-life of the reaction?
Correct Answer
Option D is the correct answer.
Detailed Explanation
For first order: k = (1/t) ln([A]₀/[A]). If 25% complete, [A] = 0.75[A]₀. k = (1/42) ln(1/0.75) = (1/42) × 0.2877 = 0.00685 min⁻¹. t½ = 0.693 / k = 0.693 / 0.00685 = 101.2 min.
Hint
Use the integrated rate law to find k, then t½ = 0.693/k.