A solution of Compound X has a transmittance of 15.8% at 475 nm in a 2.00 cm cel

Question

A solution of Compound X has a transmittance of 15.8% at 475 nm in a 2.00 cm cell. The molar absorptivity of Compound X at this wavelength is 5.47 times 10^4 L/mol middot cm. What is the concentration of compound X in the solution? a. 1.95 times 10^-6 M b. 3.92 times 10^-6 M c. 7.32 times 10^-6 M d. 7.84 times 10^-6 M At 450 and 575 nm, the indicator species H In and In-have the tabulated molar absorptivities. An unknown solution of this indicator had measured absorbance of 0.120 at 450 nm and 0.040 at 575 nm in a 1.00-cm cell. Calculate the total concentration of the indicator. a. 1.03 times 10^-5 M b. 1.03 times 10^-6 M c. 3.30 times 10^-5 M d. 2.06 times 10^-5 M

Explanation / Answer

4. Concentration = absorbance/molar absorptivity x cell length

absorbance = 2 - log(%T)

                    = 2 - log(15.8)

                    = 0.80

So,

concentration = 0.80/5.47 x 10^4 x 2

                       = 7.32 x 10^-6 M

Answer : c. 7.32 x 10^-6 M

5. absorbance of solution is equal to sum total of absorbances of al consitutuents in solution

So,

at 450 nm

0.120 = 650[HIn] + 21500[In-]

at 575 nm

0.040 = 7200[HIn] + 975[In-]

solving the two equations,

[HIn] = 4.82 x 10^-6 M

[In-] = 5.43 x 10^-6 M

Total concentration = [HIn] + [In-] = 1.03 x 10^-5 M

Answer : a. 1.03 x 10^-5 M

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