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The reaction 2X → B is a zeroth order reaction. If the initial concentration of X is 0.2 M, the half-life is 6 h. When the initial concentration of X is 0.5 M, the time required to reach its final concentration of 0.2 M will be:

1. 18.0 h
2. 9.0 h
3. 7.2 h
4. 12.0 h

Correct answer: 18.0 h

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