The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution: HIn color1 m H1 1 In2 color2 The following absorbance data were obtained for a 5.00 3 1024 M solution of HIn in 0.1 M NaOH and 0.1 M HCl. Measurements were made at wavelengths of 485 nm and 625 nm with 1.00-cm cells. 0.1 M NaOH A485 5 0.075 A625 5 0.904 0.1 M HCl A485 5 0.487 A625 5 0.181 In the NaOH solution, essentially all of the indicator is present as In2; in the acidic solution, it is essentially all in the form of HIn. (a) Calculate molar absorptivities for In2 and HIn at 485 and 625 nm. (b) Calculate the acid dissociation constant for the indicator if a pH 5.00 buffer containing a small amount of the indicator exhibits an absorbance of 0.567 at 485 nm and 0.395 at 625 nm (1.00-cm cells). (c) What is the pH of a solution containing a small amount of the indicator that exhibits an absorbance of 0.492 at 485 nm and 0.245 at 635 nm (1.00-cm cells)? (d) A 25.00-mL aliquot of a solution of purified weak organic acid HX required exactly 24.20 mL of a standard solution of a strong base to reach a phenolphthalein end point. When exactly 12.10 mL of the base was added to a second 25.00-mL aliquot of the acid, which contained a small amount of the indicator under consideration, the absorbance was found to be 0.333 at 485 nm and 0.655 at 625 nm (1.00-cm cells). Calculate the pH of the solution and Ka for the weak acid. (e) What would be the absorbance of a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 3 1024 M in the indicator and was buffered to a pH of 6.000?
0 The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution:
HIn color1 m H1 1 In2
color2
The following absorbance data were obtained for a 5.00 3 1024
M solution of HIn in 0.1 M NaOH and
0.1 M HCl. Measurements were made at wavelengths of 485 nm and 625 nm with 1.00-cm cells.
0.1 M NaOH A485 5 0.075 A625 5 0.904
0.1 M HCl A485 5 0.487 A625 5 0.181
In the NaOH solution, essentially all of the indicator is present as In2; in the acidic solution, it is essentially
all in the form of HIn.
(a) Calculate molar absorptivities for In2 and HIn at 485 and 625 nm.
(b) Calculate the acid dissociation constant for the indicator if a pH 5.00 buffer containing a small amount
of the indicator exhibits an absorbance of 0.567 at 485 nm and 0.395 at 625 nm (1.00-cm cells).
(c) What is the pH of a solution containing a small amount of the indicator that exhibits an absorbance of
0.492 at 485 nm and 0.245 at 635 nm (1.00-cm cells)?
(d) A 25.00-mL aliquot of a solution of purified weak organic acid HX required exactly 24.20 mL of a standard solution of a strong base to reach a phenolphthalein end point. When exactly 12.10 mL of the base
was added to a second 25.00-mL aliquot of the acid, which contained a small amount of the indicator
under consideration, the absorbance was found to be 0.333 at 485 nm and 0.655 at 625 nm (1.00-cm
cells). Calculate the pH of the solution and Ka for the weak acid.
(e) What would be the absorbance of a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 3 1024 M
in the indicator and was buffered to a pH of 6.000?
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