PHYSICAL CHEMISTRY IN BRIEF

CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 420

calledabsorbance, and the ratio

T=

I′

I

(12.18)

calledtransmittance.
U Main units:molar absorption coefficient: m^2 mol−^1 , absorbance and transmittance are dimen-
sionless quantities.

Example

Light of the wavelength 600 nm passes through a measuring cell which is 5 mm thick. The

solution in the cell contains CuSO 4 of the concentrationc= 10mol m−^3. The intensity of the

light passing through the cell drops to 30 per cent. What is the molar absorption coefficient?

Solution

From equation (12.16) it follows that

ε=−

log(I′/I)

c `

=

log(0.30)

10 mol m−^3 × 0. 005 m

= 10. 46 m^2 mol−^1.

The Lambert-Beer law describes the absorption of light on the macroscopic level. However,

it cannot explain why the light is absorbed by the substance. For this purpose the system has

to be described as a set of molecules. This microscopic approach is applied in section12.3.