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Homework Help: Find molar absorptivity given the following

  1. Jul 22, 2008 #1


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    1. The problem statement, all variables and given/known data

    Calculate the molar absorptivity of K2Cr2O7 at 455 nm given that 36.5 mg disolved in 500.0 mL exhibits 12.0% transmittance at 455 nm in a 2.000 cm cell.

    2. Relevant equations

    Beers law E = A/bc

    3. The attempt at a solution

    I was able to do the following part ( i solved for concentration of K2Cr2O7 )
    I found the atomic mass of K2Cr2O7
    [(0.0365 g)/(294.07 g/mol)] / (0.5000 L) = 2.482 x 10^-4

    Now I plug this into beers law but the question asks for something about a 12.0% transmittance. I am stuck here. How does transmittance relate to molar absorptivity?

    I basically have 2 unknowns in one equation... what do I need to do next?

    E = A / [(2.000 cm)(2.482 x 10^-4)] and I need to solve for A.
  2. jcsd
  3. Jul 22, 2008 #2


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    Does A = -log(.12)= .0921 ???

    and if this is the case

    E = 0.0921 / [(2.000 cm)(2.482 x 10^-4)] = 185.5359 ??? are sig figs correct as well?
    Last edited: Jul 22, 2008
  4. Jul 22, 2008 #3


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    Which number in your calculation has the fewest sig figs? How many are there? How many are in your answer?
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