JEE Main · 2004 · 24 Apr 2004 · Offline · Chemical Kinetics

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In first order reaction, the concentration of the reactant decreases from 0.8 M to 0.4 M in 15 minutes. The time taken for the concentration to change from 0.1 M to 0.025 M is

(A)30 minutes✓ Correct
(B)60 minutes
(C)7.5 minutes
(D)15 minutes

Answer: (A)

Solution

For a first-order reaction the half-life is fixed and does not depend on the starting amount. The fall from

0.8 M

0.4 M

halves the concentration once in

15

minutes, so that is one half-life. Going from

0.1 M

0.025 M

is a fall to one-quarter, two successive halvings,

0.1 \to 0.05 \to 0.025

. Two half-lives of

15

minutes each total

30

minutes.

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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