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A first order reaction has a specific reaction rate of 10⁻² s⁻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

Correct answer: 138.6 sec

Solution

For a first-order reaction, the integrated rate law is t = (2.303/k) * log(R₀/R). Substituting k = 10⁻² s⁻¹, R₀ = 20 g, and R = 5 g, we get t = (2.303/10⁻²) * log(20/5) = 138.6 seconds.

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