RE-NEET 2026 Chemistry Reaction Order MCQ Question

For a zero-order reaction, the rate law is given by:

$[A] = [A]_0 - kt$

where $[A]_0$ is the initial concentration, $k$ is the rate constant, and $t$ is the time.

In this case, the initial concentration $[A]_0 = 2 \, \text{M}$ and $k = 1.0 \, \text{mol L}^{-1} \text{min}^{-1}$. To find the time taken to complete 75% of the reaction, we need to determine when $[A] = 0.5 \, \text{M}$ (since 75% of 2 M is 1.5 M consumed).

Setting up the equation:

$0.5 = 2 - (1)(t)$

Solving for $t$ gives:

$t = 2 - 0.5 = 1.5 \, \text{min}$

However, to find the time for 75% completion, we actually need to consider the total time for the reaction to reach 0 M, which is $2 \, \text{min}$. Thus, 75% completion occurs at $0.75 \, \text{min}$.

The other options fail because they either miscalculate the time based on incorrect assumptions about the reaction order or the concentration changes.