Question

100 The IR (infrared) spectra of two pure compounds 90 (0.020 M compound A in solvent and 0.020 M compound B 80 in solvent) are given. The pathlength of the cell is 1.00 cm. 970 The y-axis in the spectra is transmittance rather than 60 50 absorption, so that the wavenumbers at which there is a dip 40 in the curve correspond to absorption peaks 2976 cm 30 3030 cm Pure A A mixture of A and B in unknown concentrations gave a percent transmittance of 46.0% at 2976 cm- and 38.6% at 3030 cm 20 Pure B 10 0 2990 3040 2940 2890 Wavenumber (cm) Wavenumber 0.020 M A 0.020 M BUnknown 3030 cm 2976 cm 38.6% 35.0% 93.0% 46.0% 76.0% 42.0% What are the concentrations of A and B in the unknown sample? [AJ М [B] Transmittance (%) M

Answer 1

formulas :

1) A = E b C

2) A = -log T

wave number	0.020 A	0.020 B	unknown
3030	A = 0.456	A = 0.0315	0.413
2976	A = 0.119	A = 0.377	0.337

molar extinction coefficient

wave number	A	B
3030	23	1.6
2976	6	19

equation:

0.413 = 23 A + 1.6 B -----------------> (1)

0.337 = 6 A + 19 B ------------------(2)

by solving these two equation we get A and B values

A = 0.0171 M ---------------------> answer

B = 0.0123 M ---------------------> answer