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A 0.023 M solution of a weak acid (HA) has a pH of 4.18. What is the Ka of the acid?
Please help will give 10 points.
1 Answer
- lenpol7Lv 77 years ago
pH = -log(10 [ H+]
The mathematical inverse is
[H+] = 10^(-pH)
Hence [H+] = 10^(-4.18)
Weak acid dissociates as HA <====---> H+ + A-
Since it dissociates in to equal numbers of cations and anions. We can say
[H+] = [A-]
Hence the [ A-] = 10^(-4.18)
The general eq'n for Ka is
Ka = [H+][A-] / [HA]
Substituting in the number.
Ka = 10^-4.18 x 10^-4.18 / 0.023
Number ' crunching'.
Ka = 10^-8.36 / 0.023
Ka = 1.89789 x 10^-7 = 0.000000189789