Question

K Question 3 of 5 > 10 The IR (infrared) spectra of two pure compounds (0.020 M compound A in solvent and 0.020 M compound B in solvent) are given. The pathlength of the cell is 1.00 cm. 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 52.7% at 2976 cm-1 and 38.2% at 3030 cm-1. 5 4 20 10 Wavenumber | 0.020 M Al 0.020 M B Unknown m" | 35.0% | 93.0% | 38.2% 76.0% | 42.0% | 52.7% | | 2976 cm-1 What are the concentrations of A and B in the unknown sample?

Answer 1

wave number (cm-1)	A	B	unknown
3030	0.456	0.0315	0.4179
2976	0.119	0.377	0.278

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

0.4179 = 23 x 1 x A + 1.6 x 1 x B

0.4179 = 23 A + 1.6 B ----------------------(1)

0.278 = 6 x 1 x A + 19 x 1 x B

0.278 = 6 A + 19 B ---------------------(2)

by solving these two

A = 0.0175 M

B = 0.00909 M