COMEDK 2025 Evening Solved Paper

Concept:The correct statement is about the relationship between completion time and half-life for a first-order reaction.Explanation:For a first-order reaction, the half-life is $t_{1/2} = \frac{\ln 2}{k}$.The time for $\frac{3}{4}^{\text{th}}$ completion (when $[A]$ drops to $\frac{1}{4}$ of initial) is $t_{3/4} = \frac{\ln 4}{k} = \frac{2 \ln 2}{k} = 2 t_{1/2}$.Thus, $t_{3/4}$ is exactly twice the half-life.Option A is false: For a zero-order reaction, the plot of $[A]$ vs. $t$ is a straight line with negative slope, not horizontal.Option C is false: For a second-order reaction, rate $\propto [A]^2$. If $[A]$ is tripled, rate becomes $3^2 = 9$ times, not 27.Option D is false: Molecularity is defined only for elementary reactions and is always an integer; order can be fractional or zero, so molecularity is not always greater than order.Answer:B. For a first order reaction the time taken for $\frac{3}{4}^{\text{th}}$ completion is 2 times the half-life period.