alleryA first-order reaction is 25% complete in 20 min. Time for 75% completion:
A40 min
B60 min
C80 min
D20 min
Answer & Solution
Correct answer: C. 80 min
We have k = (1/20) ln(1/0.75).
So k = (1/20) ln(4/3) ≈ 0.0144 min⁻¹.
For 75% complete, [A]/[A]₀ = 0.25: t = (1/k) ln 4 = ln 4 / ln(4/3) × 20 ≈ 1.386/0.288 × 20 ≈ 80 min. (Equivalently, 75% complete is twice the 'doublings' of 25% via geometric progression.)
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