* 14-10 The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution: HIn color 1 H* + In color 2 The following absorbance data were obtained for a 5.00 X 10¬4M 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 A 485 0.075 A625 = 0.904 0.1 М HСІ A 485 0.487 A625 = 0.181 In the NaOH solution, essentially all of the indicator is present as In ; in the acidic solution, it is essentially all in the form of HIn. (a) Calculate molar absorptivities for In 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 stan- dard 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). Ca ulate the pH of the solution and K, the wea acid. (e) What would be the absorbance of a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 × 10-4 M in the indicator and was buffered to a pH of 6.000?

Question
* 14-10 The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution:
HIn
color 1
H* + In
color 2
The following absorbance data were obtained for a 5.00 X 10¬4M 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
A 485
0.075
A625
= 0.904
0.1 М HСІ
A 485
0.487
A625
= 0.181
In the NaOH solution, essentially all of the indicator is present as In ; in the acidic solution, it is essentially
all in the form of HIn.
(a)
Calculate molar absorptivities for In 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 stan-
dard 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). Ca
ulate the pH of the solution and K,
the wea
acid.
(e)
What would be the absorbance of a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 × 10-4 M
in the indicator and was buffered to a pH of 6.000?
Expand
Transcribed Image Text

* 14-10 The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution: HIn color 1 H* + In color 2 The following absorbance data were obtained for a 5.00 X 10¬4M 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 A 485 0.075 A625 = 0.904 0.1 М HСІ A 485 0.487 A625 = 0.181 In the NaOH solution, essentially all of the indicator is present as In ; in the acidic solution, it is essentially all in the form of HIn. (a) Calculate molar absorptivities for In 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 stan- dard 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). Ca ulate the pH of the solution and K, the wea acid. (e) What would be the absorbance of a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 × 10-4 M in the indicator and was buffered to a pH of 6.000?

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