NEET 2025 — Chemistry solution

NEET2025Chemistry

If the rate constant of a reaction is $0.03$ s $^{- 1}$ , how much time does it take for $7.2$ mol L $^{- 1}$ concentration of the reactant to get reduced to $0.9$ mol L $^{- 1}$ ?

(Given: $lo g 2 = 0.301$ )

A
69.3 s
B
23.1 s
C
210 s
D
21.0 s

Solution

Rate constant has units of s $^{- 1}$ , so the reaction is first-order.

Going from 7.2 to 0.9 is a factor of $7.2/0.9 = 8 = 2^{3}$ , i.e. three half-lives.

$t_{1/2} = \frac{0.693}{k} = \frac{0.693}{0.03} = 23.1$ s.

$t = 3 t_{1/2} = 3 \times 23.1 = 69.3$ s.

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