Question

Substance H has maximum absorbance at 550 nm; Substance M has maximum absorbance at
650 nm. A standard solution of H has a concentration 74.2 ppm and an absorbance of 0.972 at
500 nm and an absorbance of 0.276 at 650 nm. A standard solution of M has a concentration
of 35.9 ppm and an absorbance of 0.102 at 500 nm and of 0.564 at 650 nm. A sample solution
that is a mixture of H and M has an absorbance of 0.756 at 500 nm and 0.818 at 650 nm. What
is the concentration of H and of M in the sample solution?

Answer 1

Lets tabulate the given data for readability.

	(nm)	concentration, c (ppm)	Absorbance, A
H	= 500	=74.2	0.972	0.0131
	650	74.2	0.276	0.00372
M	500	=35.9	0.102	0.00284
	650	35.9	0.564	0.0157
Mixture	500	-	0.756	-
	650	-	0.818	-

Let l be the standard path length of 1 cm.

From Beer's Lambert law, the Absorbance, concentration and path length are related as

Hence, we can calculate the different values of molar extinction coefficient .

For H, at = 500

Similarly,

For the mixture Total absorbance at a particular wavelength is given by at = 500

..........(1)

Where

are the concetrations of H and M in the mixture repsectively.

Similarly, at 650

.......(2)

Now, we have two equation (1) and (2) to solve for the two unknown concentrations.

From equation (2)

............(3)

Substituting the above in equation (1).

Hence, the concentration of M in the mixture is 40.5088 ppm.

Now, using equation (3)

Hence, the concentration of H in the mixture is 48.9279 ppm.

Hence, the concentration of H and M in the mixed sample is 48.93 ppm and 40.51 ppm respectively.