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Homework Help: Absorbance of a solution

  1. Dec 4, 2012 #1
    1. The problem statement, all variables and given/known data

    2. A 5.00 mL aliquot of a solution that contains 5.94 ppm iron (III) is treated with an appropriate excess of KSCN and is diluted to 50.0 mL.What is the absorbance of the resulting solution at 580 nm in 2.50-cm cell?

    2. Relevant equations

    3. The attempt at a solution

    I'm having trouble finding the concentration of the solution. I'm not really good at using ppm. How do I convert this to concentration? If I have the concentration, I can finish the problem by plugging in ε,b and c.
  2. jcsd
  3. Dec 4, 2012 #2
    Not sure if correct but heres what I did to find the concentration.

    5.49 ppm = 5.49 micrograms/mL = 0.00549 grams/L

    0.00549 grams/L /(55.845 grams/mole) = 0.000106 M

    0.000106 M * 5 mL / 50 mL = 0.0000106 M

    EDIT : Also I just realized that the molar absorptivity of FeSCN is not given in the book, so am I doing the problem wrong or do I need to find this online?
    Last edited: Dec 4, 2012
  4. Dec 4, 2012 #3


    User Avatar

    Staff: Mentor

    Shortcut - ppm usually means mg per L.

    1.06e-4 M is OK.
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