What is the pH? Help me.

Question

What is the pH of a solution that is 0.0100M in chloroacetic acid, CH2ClCOOH, and 0.00150M in NaCH2 ClCOO?

Answer 1

The Ka of chloroacetic acid would be most helpful.  Contrary to popular belief, I don't carry that one around in my head.  

Since you want a pH, consider the reaction (Cl-HOAc)=acid   in equilibrium with (Cl-OAc)-  and H+.   Sodium salts are ionic, so the [Cl-OAc]-  would be essentially 1.5x10-3 mole/L.  Since there is a high content of the ion, the dissociation of the acid is suppressed even more than it would be if the solution just had only acid.  So the [Cl-HOAc] is essentially 1x10-2 mole/L.

   Then [1.5x10-3][H+]/[1x10-2] = Ka.  and [H+]=6.7Ka.  

Answer 2

Ka = 1.54x10^-3 or 1.38x10^-3 depending on where I look.

The solution is a buffer composed of the salt of a weak acid and the acid. Use the Henderson-Hasselbalch equation-

pH = pKa + log(A-/HA)

pH = pKa + log(0.0015/0.0100)

pH = pKa - 0.82

pH = 2.81 - 0.82 = 1.99 (if Ka = 1.54x10^-3)

pH = 2.86 - 0.82 = 2.04 (if Ka = 1.38x10^-3)