How to calculate transmittance from concentration?

Im not sure how to do these

3. A 10 mM solution has a transmittance of 45%. Calculate the transmittance of a 20mM solution at the same wavelength and optical pathlength.

4. Two colored solution, C and D contain different dyes. Let the transmittances of these two solutions be TC and TD respectively, when measured in a 1-cm cell at a given wavelength. Express the transmittance TCD of the mixture of equal volumes of C and D, measured under the same conditions, in terms of TC and TD.

1 Answer

  • 8 years ago
    Favorite Answer

    First question...

    Using the Beer-Lambert Law,

    A = ebc and A = -log T


    **e = molar absorptivity (which is wavelength-dependent)

    **b = optical pathlength

    **c = concentration

    **T = transmittance

    Equating both A...

    ebc = -log T [Eq.1]

    or eb = (-log T)/c

    If in both cases they have the same b and same e...

    Thus, (-log T)/c of the first case is equal to (-log T)/c of the second case

    (-log 0.45)/10 = (-log T)/20

    >>> T = 20.25%

    Second question...

    We use the equation:

    A(sol'n) = A(C) + A(D)

    -log TCD = -log TC + -log TD

    -log TCD = -log (TC*TD)

    >>> TCD = TC*TD

    Hope that helps! :)

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