If half-life period for a first order reaction in A is 2 minutes, how long will it take [A] to reach 10 % of its initial concentration?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 6.65 min
For a first order reaction the half-life is independent of concentration: k = 0.693/t1/2.
k = 0.693 / 2 min = 0.3465 min−1
[A] falls to 10% of its initial value, so [A]0/[A] = 100/10 = 10:
t = (2.303/k) log ([A]0/[A])
t = (2.303 / 0.3465) × log 10
t = 6.646 × 1 ≈ 6.65 min
NCERT (2026-27 edition):
Half-Life of a Reaction – …
