The figure at the right shows the absorption spectra of two pure compounds, A an

Question

The figure at the right shows the absorption spectra of two pure compounds, A and B, each at a concentration of 1.50*10^-4. The table below shows the measured absorbance of these standards in a 1.000-cm pathlength cell at five different wavelengths, along with the absorbance of an unknown mixture of the two compounds at the same wavelengths.

Compand A

Standard

Compound B

Standard

0.642

Use a spreadsheet to calculate the concentration of each compound in the mixture. [A]=? [B]=?

Wavelength

Compand A

Standard

Compound B

Standard

Mixture 440 0.430 0.213 0.263 470

0.642

0.367 0.426 500 0.609 0.444 0.471 530 0.392 0.457 0.422 560 0.231 0.334 0.293

Explanation / Answer

Calculate molar absorptivity at 470 nm and 500 nm

A = ebc

e = molar absorptivity

b = 1 cm

c = concentration

470 nm

For A: e = 0.642/1.5 x 10^-4 = 4280

For B : e = 0.367/1.5 x 10^-4 = 2446.67

At 500 nm

For A: e = 0.609/1.5 x 10^-4 = 4060

For B : e = 0.444/1.5 x 10^-4 =2960

For unknown at 470 nm,

A = A(A) + A(B)

0.426 = 4280[C(A)] + 2446.67[C(B)]

For unknow at 500 nm,

0.471 = 4060[C(A)] + 2960[C(B)]

solve for A and B,

concentration of A = 3.97 x 10^-5 M

concentration of B = 1.05 x 10^-4 M

In the mixture

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