NEET Chemistry Chemical Kinetics Questions Solved


Two substances A and B are present such that [A0] = 4[B0] and half life of A is 5 minute and that of B
is 15 minute. If they start decaying at the same time following first order kinetics how much time later will the concentration of both of them would be same.

(A) 15 minute                                        

(B) 10 minute

(C) 5 minute                                          

(D) 12 minute

(A) Amount of A left in n1 halves = 12n1 [A0]

Amount of B left in n2 halves = 12n2 [B0]

At the end, according to the question

A02n1=B02n242n1=12n2,A0=4B02n12n2=42n1-n2=22n1-n2=2n2=n1-2                                          .....1Also t=n1×t1/2A; t=n2×t1/2B

(Let concentration of both become equal after time t)

n1×t1/2An2×t1/2B=1n1×5n2×15=1n1n1=3          .....2

For equation (1) and (2)

n1=3, n2=1t=3×5=15 minutes

Difficulty Level:

  • 35%
  • 32%
  • 11%
  • 25%
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