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Half-life of a reaction
Concepts tested here
- Number of half-lives
All Questions
2011 AIPMT-MAINS 1 question
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The half life of a substance in a certain enzyme-catalysed reaction is 138 s. The time required for the concentration of the substance to fall from 1.28 mg $L^{-1}$ to 0.04 mg $L^{-1}$ isThe reaction is treated as first order, for which the half-life is constant.
$\frac{\text{final concentration}}{\text{initial concentration}} = \frac{0.04}{1.28} = \frac{1}{32} = \left(\frac{1}{2}\right)^5$
Step by step: 1.28 → 0.64 → 0.32 → 0.16 → 0.08 → 0.04, which is five half-lives.
Time required = $5\times t_{1/2} = 5\times138$
= 690 s