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juliaanne82 juliaanne82
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11 years ago
How would I calculate the pH of a solution of a weak acid if I only know the molarity of the acid? Don't I need a Ka value? Ex: Determine the pH of a 0.531M HF solution.
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#1
11 years ago
Yes you need the Ka value since the formula is

pH= (pKa-logc)/2  where pKa= - log Ka

From my source you see that pKa already given is 3.15
c=0.531 (monoacid) so log c =-0.275

pH= (3.15-(-0.275))/2 = 1.712
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#2
11 years ago
Find on : Ka table weak acids the Ka of HF

Ka = x^2 / 0.531-x
x = [H+]
pH = - log x
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#3
11 years ago
Another approach, which also requires the Ka for the weak acid, is to start with the equilibrium expression for the ionization of the weak acid:

HF Leftwards Arrow> H+ + F-

Ka = [H+][F-]/[HF]

Since you are beginning with just HF, [H+] = [F-]. So, the expression for Ka becomes:

Ka = 3.53 X 10^-4 = [H+}^2/0.531
[H+]^2 = 1.87 X 10^-4
[H+] = 0.0137
pH = 1.86
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