Two different species, A and B, coexist in the same solution. You have prepared

Question

Two different species, A and B, coexist in the same solution. You have prepared four solutions, each contain A and B but at different concentrations. The concentration of each species varies for your four samples according to the table below. You measured the percent transmission at 610 nm for each sample in a 10 mm by 10mm cuvette. What is the molar absorptivity for A and B?

#                              A[M]                B[M]                         % transmission

1                             5e-5                 2e-4                             13.72

2                             1e-4                 1.5e-4                          18.84

3                             1.5e-4              1e-4                             25.85

4                             2e-4                 5e-5                             35.48

Explanation / Answer

4000 cm-1moles-1L = e2 for B

1250 cm-1moles-1L = e2 for A

The total absorbance equals the contribution from each species. Beer%u2019s law is A=ebc, with b = 1 cm and one needs to consider the contribution from each species.

At= e1bc1+ e2bc2

Use the data to solve for one component of the molar absorptivity.

0.8625= e15E-5+ e22E-4

0.725= e11E-4+ e21.5E-4

Normalize for e1 to solve for e2by multiplying the top equation by 2

1.725= e11E-4+ e24E-4

-[0.725= e11E-4+ e21.5E-4]

1.000=e22.5E-4

1/2.5E-4 = 4000 cm-1moles-1L = e2 for B

Insert this an equation and solve for e1

0.725= e11E-4+ 4000*1.5E-4

0.125= e11E-4

1250 cm-1moles-1L = e1 for A

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