The IR (infrared) spectra of two pure compounds (0.020 M compound A in solvent,

Question

The IR (infrared) spectra of two pure compounds (0.020 M compound A in solvent, and 0.020 M compound B in solvent) are shown to the right. The pathlength of the cell is 1.00 cm. Note that the y axis in the spectra is transmittance rather than absorption, so that the wavenumbers at which there is a dip in the curve correspond to absorption peaks 100 90 80 70 60 50 40 2976 cm1 A mixture of A and B in unknown concentrations gave a percent transmittance of 48.2% at 2976 cm-1 and 41.3% at 3030 cm-1 3030 cm1 20 _ Pure A Pure B 3040 2990 2940 2890 Wavenumber 0.020 MA 0.020 M B 3030 cm-1 | 35.0% 2976 cm What are the concentrations of A and B in the unknown sample? Unknown 41.3% 48.2% Wavenumber (cm-1) 93.0% | 76.0% 42.0% Number Number M A M B

Explanation / Answer

Absorbance = A = E x b x C

0.384 = 23 x 1 x A + 1.6 x 1 x B

0.384 = 23 A + 1.6 B ----------------------(1)

0.317 = 6 x 1 x A + 19 x 1 x B

0.317 = 6 A + 19 B ---------------------(2)

by solving these two

A = 0.0159 M

B = 0.0117 M

wave number (cm-1) A B unknown 3030 0.456 0.0315 0.384 2976 0.119 0.377 0.317

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