1. In the laboratory, a general chemistry student measured the pH of a 0.540 M a
ID: 1041193 • Letter: 1
Question
1. In the laboratory, a general chemistry student measured the pH of a 0.540 M aqueous solution of acetylsalicylic acid (aspirin), HC9H7O4 to be 1.880.
Use the information she obtained to determine the Ka for this acid.
Ka(experiment) =
2. In the laboratory, a general chemistry student measured the pH of a 0.540 M aqueous solution of hydrofluoric acid to be 1.691.
Use the information she obtained to determine the Ka for this acid.
Ka(experiment) =
3.In the laboratory, a general chemistry student measured the pH of a 0.540 M aqueous solution of benzoic acid, C6H5COOH to be 2.249.
Use the information she obtained to determine the Ka for this acid.
Ka(experiment) =
Explanation / Answer
1)
use:
pH = -log [H+]
1.88 = -log [H+]
[H+] = 1.318*10^-2 M
HC9H7O4 dissociates as:
HC9H7O4 -----> H+ + C9H7O4-
0.54 0 0
0.54-x x x
Ka = [H+][C9H7O4-]/[HC9H7O4]
Ka = x*x/(c-x)
Ka = 1.318*10^-2*1.318*10^-2/(0.54-1.318*10^-2)
Ka = 3.299*10^-4
Answer: 3.30*10^-4
2)
use:
pH = -log [H+]
1.691 = -log [H+]
[H+] = 2.037*10^-2 M
HF dissociates as:
HF -----> H+ + F-
0.54 0 0
0.54-x x x
Ka = [H+][F-]/[HF]
Ka = x*x/(c-x)
Ka = 2.037*10^-2*2.037*10^-2/(0.54-2.037*10^-2)
Ka = 7.986*10^-4
Answer: 7.99*10^-4
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