Determine Ka from pH?

Question

If I have a 8.345x10-3 moles in .05L of solution, I have .1669M right? Now, if the pH is 4, then is the formula for Ka is:
(10x-4)(10x-4)/.1668?

Then how do I calculate the Ka after I've added, say, .005L of a base of .4M?

The acid is a weak acid and the base is a strong base (NaOH). The Kb is unknown.

Answer 1

You are not giving us all of the info.
Yes the total or initial concentration of the acid is C=(8.345*10^-3)/0.05 = 0.1669 M

For a monoprotic acid Ka=[H+][A-]/{HA]
[A-]=[H+]=10^-pH=10^-4
[HA]=C-[H+]= 0.1669-0.0001= 0.1668
so you have calcualted Ka correctly.

Ka depends on the nature of the acid and temperature (for small molecules whereas for bigger molecules like proteins it is also affected by the nature of the surrounding chemical groups).

Thus addition of base doesn't affect Ka, only the pH.
Is the base monoprotic or not? If not, what's the valency? Is it strong or weak? If it's weak what's the Kb? Is it the conjugate base of the weak acid? Without these info we can't calculate the pH.