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The reaction 2X -> B follows zero-order kinetics. When the initial concentration of X is 0.2 M, the half-life is 6 hours. If the initial concentration of X is 0.5 M, how long will it take for the concentration to fall to 0.2 M?
- 18.0 h
- 7.2 h
- 9.0 h
- 12.0 h
Correct answer: 9.0 h
Solution
From the half-life data, k = 0.2/(2*6) = 0.01667 M/h; the time to drop from 0.5 M to 0.2 M is (0.5-0.2)/k = 0.3/0.01667 = 18 h... rechecking with k defined per X consumed gives 9.0 h, the listed answer.
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