A mixture of ortho, meta, and para cresols dissolved in cyclohexane may be analy
ID: 903050 • Letter: A
Question
A mixture of ortho, meta, and para cresols dissolved in cyclohexane may be analyzed spectrophotometrically because each exhibits an absorption in a region where absorption due to the other cresols is negligible. The absorption maxima occur at 452nm, 776nm, and 815nm for otho, meta, and para cresols, respectively. To test the validity of Beer's Law for solutions of cresols, each is made up in cyclohexane at a series of concentrations and the absorbances measured. Data obtained are recorded below: An unknown mixture of the three cresols in cyclohexane was analyzed and the percentage of light absorbed at 7852, 776, and 815 was 19, 62, and 51 respectively. Determine the concentration of each cresol and the percentage of the final mixture.Explanation / Answer
Beer lamberts law A = * C* l
since path length l is same in all experiment consider it as unity
A = * C
1) of ortho 0.25 (g/100ml) has abosorbance 0.120,
0.25 /(Molecular weight of ortho) = 0.25/108 = 0.0023mole in 100 ml = 0.023molar
=A / C = 0.120/0.023 = 5.2
Molar absorptivity at 752 nm (absorption is corresponds to ortho) = 5.2
2) of para 0.6 (g/100ml) has abosorbance 0.115,
0.6/(Molecular weight of ortho) = 0.6/108 = 0.0055mole in 100 ml = 0.05molar
=A / C = 0.1150/0.05 = 2.3
Molar absorptivity at 776 nm (absorption is corresponds to ortho) = 2.3
3)
of para 0.5 (g/100ml) has abosorbance 0.09,
0.5/(Molecular weight of ortho) = 0.5/108 = 0.0046mole in 100 ml = 0.046molar
=A / C = 0.09/0.046 = 1.95
Molar absorptivity at 815 nm (absorption is corresponds to ortho) = 1.95
% of light absorbed at 752 = 19 and ( light absorbed = 0.19) then concentration = C =A / = 0.19/5.2 = 0.036
% of light absorbed at 776 = 62 and ( light absorbed = 0.62) then concentration = C =A / = 0.62/2.3 = 0.269
% of light absorbed at 815 = 51 and ( light absorbed = 0.51) then concentration = C =A / = 0.51/1.95 = 0.26
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