Mapt Sapling Learning The IR (infrared) spectra of two pure compounds (0.020 M c
ID: 102899 • Letter: M
Question
Mapt Sapling Learning 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 2976 cm A mixture of A and B in unknown concentrations gave a percent transmittance of 48.2% at 2976 cm-1 and 36.0% at 3030 cm-1 F 30 20 10 3030 cm- Pure A Pure B 3040 2990 2940 2890 Wavenumber 0.020 MA 0.020 M B 3030 cm-1 | 35.0% 2976 cm-1 | 76.0% What are the concentrations of A and B in the unknown sample? Unknown 36.0% 48.2% Wavenumber (cm- 93.0% 42.0% Number [B] = 110 M A M BExplanation / Answer
wave number (cm-1)
A
B
unknown
3030
0.456
0.0315
0.444
2976
0.119
0.377
0.317
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
0.444 = 23 x 1 x A + 1.6 x 1 x B
0.444 = 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 = 0.0186 M
B = 0.0108 M
wave number (cm-1)
A
B
unknown
3030
0.456
0.0315
0.444
2976
0.119
0.377
0.317