A first order reaction is found to have rate constant, \(k = 3.5 × 10^{-4} s^{-1}\). Its half life will be:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(198 × 10 s\)
For a first order reaction the half-life does not depend on the initial concentration and is found from the rate constant alone:
\( t_{1/2} = \displaystyle\frac{0.693}{k} \)
k = 3.5 × 10−4 s−1
t1/2 = 0.693 / (3.5 × 10−4 s−1)
t1/2 = 0.198 × 104 s = 1980 s = 198 × 10 s
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Half-Life of a Reaction – “… A first order reaction is found to have a rate constant, k = 5.5 × 10–14 s–1. Find the half-life of the reaction.”