Ncert Exercises & Solutions

A first-order reaction is 50 % completed in 1.26 × 10¹⁴ s. The time required for 100 % completion of the reaction will be:

1. 1.26 × 10¹⁵s

2. 2.52 × 10¹⁴s

3. 2.52 × 10²⁸ s

4. Infinite

HINT: $k = \frac{1}{t} \ln \frac{[A_{0}]}{[A_{t}]}$

Explanation:

The time taken for half the reaction to complete. ie., the time in which the concentration of a reactant is reduced to half of its original value is called half-life period of the reaction.

But it is impossible to perform 100% of the reaction. Whole of the substance never reacts because in every half-life, 50% of the substance reacts. Hence, time taken for 100% completion of a reaction is infinite.

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