For a first order reaction (A) → products the concentration of A changes from 0.1 M to 0.025 M in 40 minutes. The rate of reaction when the concentration of A is 0.01 M is:

1.73 × 10⁻⁵ M/min
3.47 × 10⁻⁴ M/min
3.47 × 10⁻⁵ M/min
1.73 × 10⁻⁴ M/min

Correct answer: 3.47 × 10⁻⁴ M/min

Solution

0.1 -> 0.025 M is two halvings, so 2*t_half = 40 min -> t_half = 20 min and k = 0.693/20 = 0.0347 /min. Rate at [A] = 0.01 M is k[A] = 0.0347*0.01 = 3.47x10^-4 M/min.

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