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. A mixture of A and B in unknown concentrations gave a percent transmittance of 45.9% at 2976 cm-1 and 42.6% at 3030 cm-1. Transmittance (%) o 38 8 3 3 8 3 3 2976 cm-1 3030 cm-1 — Pure A — Pure B 3040 2890 Wavenumber 0.020 M A | 3030 cm-1 35.0% 2976 cm-1 76.0% 0.020 MB 93.0% 2990 2940 Wavenumber (cm-1) Unknown 42.6% 42.0% 45.9% What are the concentrations of A and B in the unknown sample? Number Number [A]= D | M A [B] = MB

Explanation / Answer

wave number (cm-1)

A

B

unknown

3030

0.456

0.0315

0.371

2976

0.119

0.377

0.338

Absorbance = A = E x b x C

wave number

A

B

3030

E = 23 M-cm-1

1.6 M-1 cm-1

2976

E = 6.0 M-1 cm-1

19 M-1 cm-1

0.371 = 23 x 1 x A + 1.6 x 1 x B

0.371 = 23 A + 1.6 B ----------------------(1)

0.338 = 6 x 1 x A + 19 x 1 x B

0.338 = 6 A + 19 B ---------------------(2)

by solving these two

A = 4.81 x10^-3 M

B = 0.0163 M

wave number (cm-1)

A

B

unknown

3030

0.456

0.0315

0.371

2976

0.119

0.377

0.338

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