A first order reaction has a specific reaction rate of 10^-2\ sec^-1. How much time will it take for 20 g of the reactant to reduce to 5 g ?

1. 138.6 sec

2. 346.5 sec

3. 693.0 sec

4. 238.6 sec

Doubtly Asked 30 Oct 2026 NEET UG 2017 Practice test neet Save WhatsApp Report

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Answer: 1. 138.6 sec

Key idea: first-order integrated rate law \(t=\dfrac{2.303}{k}\log\dfrac{a}{a-x}\).

\(t=\dfrac{2.303}{10^{-2}}\log\dfrac{20}{5}=230.3\times0.602=138.6\ \mathrm{s}\).

Check: 20 g to 5 g is two half-lives, \(t_{1/2}=0.693/k=69.3\) s, so 138.6 s.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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