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Concentration Changes Over Time

AP Chemistry· difficulty 4/5

time (s) ln[A]

A first-order reaction has [A]0[A]_0 = 0.500 M and [A][A] = 0.125 M after 84.0 s. What is the half-life?

  • A

    t1/2=56.0t_{1/2} = 56.0 s

  • B

    t1/2=21.0t_{1/2} = 21.0 s

  • C

    t1/2=42.0t_{1/2} = 42.0 s

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  • D

    t1/2=28.0t_{1/2} = 28.0 s

Explanation

[A]/[A]0[A]/[A]_0 = 0.25 = (1/2)2(1/2)^2, so 2 half-lives in 84.0 s, giving t1/2t_{1/2} = 42.0 s.

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