You are testing a newly discovered pH indicator which has a Ka of 4.0 10-6. You
ID: 950812 • Letter: Y
Question
You are testing a newly discovered pH indicator which has a Ka of 4.0 10-6. You carefully prepare a solution of 0.0026 M indicator in solutions of 0.5 M HCl, 0.5 M NaOH, and in unknown solution A. Then you put an undetermined amount of indicator into unknown solution B. You measure the visible absorption spectra of each of the four solutions. The data is illustrated in the graph and given in the table.
What is the pH of solution A, which contains 0.0026 M indicator?
What is the pH of solution B, which contains an undetermined amount of indicator?
Explanation / Answer
Solution:
From the given data it is clear Ka ~ Ksp from this pKa can be computed to find pH,
Ksp = [NaOH][HCL][Sol.A]
4.0 E-06 = [0.5][0.5][Sol.A]
[Sol.A] = 0.000016
pH = 14 - pOH = 14- 4.79 = 9.204
in solution B indicator added is 0.0026 M ----> log (0.0026) = -2.5850
pH = 9.5850
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