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Homework Help: Beer's Law Question

  1. Aug 27, 2010 #1
    1. The problem statement, all variables and given/known data
    If I measure an absorbance at 560nm of 0.80 absorbance units for a solution of the basic form of phenolphthalein, what is the concentration of phenolphthalein?

    2. Relevant equations
    Beer's Law -- A(sub lamda)=e*b*c

    3. The attempt at a solution

    The question is asking for concentration, so I'll need to solve for c. I think A is the absorbance units, but beyond that I believe I'm missing information.

    b is supposed to be the path length, which I believe is the width of whatever cuvet is being used (the problem doesn't specify what that is). e is molar absorbtivity, but isn't that a constant?

    Is this problem able to be solved as it is or do I need more information?

  2. jcsd
  3. Aug 27, 2010 #2


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    Gold Member

    Cuvet length is 1 cm by convention. Molar absorptivity is going to vary with temperature and pH as you can see here:


    Unfortunately they don't report a value for phenolphthalein but estimating from Fig. 6, e should be about 1770/(M.cm) at 20C and basic conditions.
  4. Aug 28, 2010 #3
    Thank you for the reply. It seems there was no way to calculate an answer based on the question alone. I was worried I was missing something obvious.

    I will ask my lab instructor about it next week.
  5. Aug 28, 2010 #4


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    Staff: Mentor

    I agree 1 cm is a common cuvette size, but calling it a "convention" is too far fetched.
    Last edited by a moderator: Aug 13, 2013
  6. Aug 28, 2010 #5


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    Gold Member

    Quite right, I stand corrected.
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