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pH of weak acids and weak bases

# pH of weak acids and weak bases - Weak acids Question A...

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Weak acids Question: A 0.128 M solution of uric acid (HC 5 H 3 N 4 O 3 ) has a pH of 2.39. Calculate the K a of uric acid. Another reason for using the generic acid formula of HA is that this avoids the need to constantly write a somewhat complex formula for the anion portion of the weak acid. In fact, as I do this problem, I will write Ur¯ for the anion portion of uric acid (the C 5 H 3 N 4 O 3 ¯). Solution a) [H + ] = 10¯ pH = 10¯ 2.39 = 4.0738 x 10¯ 3 M b) [Ur¯] = [H + ] = 4.0738 x 10¯ 3 M c) I will use 0.128 M K a = [(4.0738 x 10¯ 3 ) (4.0738 x 10¯ 3 )] / 0.128 = 1.30 x 10¯ 4 Question # 2. A student prepares a 0.45 M solution of a monoprotic weak acid and determines the pH to be 3.68. What is the K a of this weak acid? Answer: K a = 9.7 x 10¯ 8 pH of WEAK BASES Question 1: Codeine (C 18 H 21 NO 3 ) is a weak organic base. A 5.0 x 10¯ 3 M solution of codeine has a pH of 9.95. Calculate the value of K b for this substance.

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Note that I will use B to symbolize the weak base. No one cares what the specific base is because the technique to be explained works for all weak bases. What happens is that some teachers will
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pH of weak acids and weak bases - Weak acids Question A...

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