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Homework Help: First Order Reaction calculation

  1. Jul 22, 2009 #1
    1. The problem statement, all variables and given/known data

    The decomposition of AB is first order with a k = 2.3 x 10-7 s-1 at 45oC. If the initial concentration is 0.25 M, the concentration after 2.3 min is:

    2. Relevant equations

    ln[A]t = -kt x ln[A]0

    3. The attempt at a solution

    I have subbed in the values I have but I am not getting the right answer. I am unable to solve the logarithmic equation and need help there but also need to know if my method is correct.

    I have subbed in:

    ln[A]t = -2.3 x 10-7 x 138 (seconds) x ln.25

    I am getting :

    ln[A]t = 4.4 x 10-5

    My problem is that I cannot remember how to solve that equation for [A]t. It's been a while since i looked at logarithmic equations. I have done google searches for the past hour and tried a few things but can't solve it. I'm not sure if my method is even correct in the first place.

    Please help.
  2. jcsd
  3. Jul 22, 2009 #2


    User Avatar

    Staff: Mentor

    What is logarithm definition?
  4. Jul 22, 2009 #3
    YES! I have figured it out. Sometimes I wonder if I have the aptitue to learn this chemistry stuff, but I am doing well so far and I am teaching myself so it's obviously not going to be easy all the way. Anyway.. this was where I left off last time.

    ln[AB]t = -2.3 x 10-7 x 138 (seconds) + ln.25
    = -1.386

    ln[AB]138 = -1.386
    [AB] = e-1.386 **This is the step that was killing me, simple logarithmic algebra)**

    [AB] = .25M

    Now, lets see how I get on with the rest of the problems!!
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