The IR (infrared) spectra of two compounds (0.010 M and 0.010 M compound B in so

Question

The IR (infrared) spectra of two compounds (0.010 M and 0.010 M compound B in solvent) are shown to the right. 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 g 70 A in solvent, The pathlength of the cell is 1.00 2976 cm A mixture of A and B in unknown concentrations gave a percent transmittance of 50.5% at 2976 cm-1 and 36.2% at 3030 cm-1 30 20 10 3030 cm Pure A Pure B 3040 2990 2940 2890 Wavenumber 0.010 MA 3030-1 135.0% 2976 crm-1 176.0% What are the concentrations of A and B in the unknown sample? 0.010 M B 93.0% 42.0% Unknown 36.2% 50.5% Wavenumber (om' Number M A M B

Explanation / Answer

wave number (cm-1)

A

B

unknown

3030

0.456

0.0315

0.441

2976

0.119

0.377

0.297

Absorbance = A = E x b x C

wave number

A

B

3030

E = 45.6 M-cm-1

3.15 M-1 cm-1

2976

E = 11.9 M-1 cm-1

37.7 M-1 cm-1

0.441 = 45.6 A + 3.15 B ----------------------(1)

0.297 = 11.9 A + 37.7 B ---------------------(2)

by solving these two

A = 0.00933 M

B = 0.00493 M

wave number (cm-1)

A

B

unknown

3030

0.456

0.0315

0.441

2976

0.119

0.377

0.297

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