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For a first order reaction with half-life t1/2 = 20 minutes, what fraction of the reactant remains after 40 minutes?

1. 1/4
2. 1/2
3. 1/8
4. 1/6

Correct answer: 1/4

Solution

40 min = 2 half-lives, so the fraction left = (1/2)² = 1/4.

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