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 50.3% at 2976 cm-1 and 40.6% at 3030 cm^-1. Wavenumber 0.020 M A 0.020 M B Unknown 3030 cm^-1 35.0% 93.0% 40.6% 2976 cm^-1 76.0% 42.0% 50.3% What are the concentrations of A and B in the unknown sample? Number Number

Explanation / Answer

A = ebc

Absorbance = 2 - log(%T)

For pure A,

at 3030 cm-1

transmittance from curve = 30%

A = 2 - log(30) = 0.523

e = 0.523/0.2 = 2.615

For pure B at 2976 cm-1,

A = 2 - log(40%) = 0.398

e = 0.398/0.2 = 1.99

b = 1

At 3030 cm-1 only A gives signal and at 2976 cm-1 only B gives signal,

Unknown at 3030 cm-1,

A = 2 - log(40.6) = 0.391

concentration of A = 0.391/2.615 = 0.15 M

Unknown at 2976 cm-1,

A = 2 - log(50.3%) = 0.298

concentration of B = 0.298/1.99 = 0.15 M

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