Which of the following can be the rate constant of a first order reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2 x 10-2 s-1
The unit of rate is always mol L−1 s−1, and since Rate = k[A]n, the unit of k is (mol L−1)1−n s−1. For a first order reaction (n = 1) the concentration part cancels:
\( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{\mathrm{mol\,L^{-1}}}=\mathrm{s^{-1}} \)
So only 2 × 10−2 s−1 can be a first order rate constant.
Shortcut: A first order rate constant has no concentration term at all. Only Option (3) has s−1 alone.
NCERT (2026-27 edition):
Order of a Reaction – …
