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Homework Help: Kinetics Question (spent over 7 hours on this)

  1. Jan 31, 2009 #1
    1. The problem statement, all variables and given/known data
    The following reaction was monitored as a function of time:
    AB---> A + B
    A plot of 1/[AB] versus time yields a straight line with slope 5.2×10−2 M \s.

    If the initial concentration of AB} is 0.210 M, and the reaction mixture initially contains no products, what are the concentrations of {A} and {B} after 80 s?

    2. Relevant equations

    1/A = kt + 1 / A initial

    3. The attempt at a solution

    1/A = (5.2x10^-2)(80 s) + 1/0.21 M
    *When I did this equation it was wrong. The correct answer was that the concentration of both A and B=9.8×10−2,9.8×10−2. Please tell me what I am doing wrong. Thanks!
  2. jcsd
  3. Jan 31, 2009 #2

    and [AB] at t=0 can be used because k is a constant ratio. The rate is the slope of the line. However, the problem appears to have an error:


    The units should be


    If 1/[AB] vs. time is linear then k is not linear. Is this a first order reaction?
  4. Jan 31, 2009 #3
    Thanks for your response. I think the rate is second order because given was: Rate=k,[AB]^2

    I just plugged in those numbers for first order and I am still getting the answer wrong. This question is frustrating me to the max. Any help would be hugely appreciated.
  5. Jan 31, 2009 #4
    Since it's a second order reaction, your equation for 1/[AB] is correct. Use conservation of mass. Your answer gives the concentration of AB after 80 seconds so this means what was converted is

    .21 - [AB] = 9.8 x 10-2 M = concentration of products.
    Last edited: Jan 31, 2009
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