Concept explainers
The acid-base indicator HIn undergoes the following reaction in dilute aqueous solution:
The following absorbance data were obtained for a 5.00 × I0-4 M solution of HIn in 0.1 M NaOH and 0.1 M HC1. Measurements were made at wavelengths of 485 nm and 625 nm with 1.00-cm cells.
-
0.1 M NaOH
A485 = 0.075
A625 = 0.904
0.1 M HC1 A485 = 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 ¡fa 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 of0.492 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-cmcells). 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 × 10-4 M in the indicator and was buffered to a pH of 6.000?
(a)
Interpretation:
Molar absorptivities for In- and HIn at 485 and 625 nm should be calculated.
Concept introduction:
The Beer-Lambert Law is:
A − absorbance
l − length of the solution light passes through (cm)
c − concentration of solution (mol/L)
Answer to Problem 14.10QAP
At 484 nm,
At 625 nm,
Explanation of Solution
At 484 nm,
At 625 nm,
(b)
Interpretation:
The acid dissociation constant of the indicator should be calculated.
Concept introduction:
The Beer-Lambert Law is:
A − absorbance
l − length of the solution light passes through (cm)
c − concentration of solution (mol/L)
Answer to Problem 14.10QAP
Explanation of Solution
At 485 nm
At 625 nm
Solving the above equations as:
(c)
Interpretation:
pH of the solution should be calculated.
Concept introduction:
The formula used to determine the pH is:
Where,
Answer to Problem 14.10QAP
Explanation of Solution
At 485 nm
At 625 nm
(d)
Interpretation:
The pH of the solution and Ka for the weak acid should be determined.
Concept introduction:
The formula used to determine the pH is:
Where,
Answer to Problem 14.10QAP
Explanation of Solution
At 485 nm
At 625 nm
Since the HX solution is half neutralized
(e)
Interpretation:
Absorbance of a solution at 485 and 625 nm should be determined.
Concept introduction:
The Beer-Lambert Law is:
A − absorbance
l − length of the solution light passes through (cm)
c − concentration of solution (mol/L)
Answer to Problem 14.10QAP
Explanation of Solution
So,
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Chapter 14 Solutions
Principles of Instrumental Analysis
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