Question

What is the pH of a solution of 1.0 x 10-10 F weak acid? Why don’t I have to tell you which weak acid it is?

Answer 1

Weak acids will not completely dissociate into ions as strong acids and strong bases do. For example if you take 100 molecules of weak acid (depending upon the nature of weak acid) let us assume 26 molecules are dissociated and 74 molecules intact. This will give lesser number of hydrogen ions in solution. We know that pH is negative logarithm of hydrogen ion concentration, this will affect on pH calculations. You wont get accurate pH values.

Let's try with 0.001 M HCl and 0.001M CH₃COOH

0.001M HCl pH = -log(0.001) = 3

but 0.001M CH₃COOH pH ≠ 3   as the dissociation constant for Acetic acid (CH3COOH) Ka=1.8 x 10^-5

[H+] = √ (Ka x concentration) = √ (1.8 x 10^-5) x (0.001) = 1.34 x 10^-4

pH = -log(1.34 x 10^-4) = 3.87

Notice that     0.001M HCl pH = 3                 and            0.001M CH₃COOH    pH = 3.87

Hence it is important to know the identity of weak acid.

Hope i have explained well to you. Thank You! :-)

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