A buffer that contains 0.35 M of a base, B and 0.28 M of its conjugate acid BH+,
ID: 494797 • Letter: A
Question
A buffer that contains 0.35 M of a base, B and 0.28 M of its conjugate acid BH+, has a pH of 9.71. What is the pH after 0.01 mol of HCl are added to 0.71 L of the solution? (Answer: 9.67)
Calculate the pH during the titration of 30.00 mL of 0.1000 M LiOH with 0.1000 M HClO4 after 30.5 mL of the acid have been added. (Answer: 3.08)
Calculate the pH during the titration of 20.00 mL of 0.1000 M HClO(aq) with 0.1000 M NaOH(aq) after 5 mL of the base have been added. Ka of hypochlorous acid = 3.0 x 10-8 (Answer: 7.05)
Explanation / Answer
Given, pH of buffer = 9.71
[H+] = 1.95 * 10-10 M
and [OH-] = (1.0*10-14 ) / [H+]
= 5.13*10-5 M
B (aq) + H2O (l) <-----> BH+ (aq) + OH- (aq)
I(M) 0.35 0.28 0
C(M) -5.13*10-5 +5.13*10-5 +5.13*10-5
-------------------------------------------------------
E(M) 0.35 0.28 5.13*10-5
Kb = [BH+][OH-] / [B]
= 0.28 * 5.13*10-5 / 0.35
= 4.104*10-5
When 0.01 mol HCl is added to 0.71 L of this solution, concentration is 0.01 mol / 0.71 L
= 0.01408 M
B + HCl ------> BH+ + Cl-
I(M) 0.35 0.01408 0.28
C(M) -0.01408 - 0.01408 +0.01408
------------------------------------
E(M) 0.336 0 0.294
Kb = [BH+][OH-] / [B]
4.104*10-5= 0.294 * [OH-] / 0.336
[OH-] = 4.69*10-5 M
Then, [H+] = 2.13*10-10 M
so pH = 9.67
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