In reply to JimSh:
The pKa should be the same in each case - it's a property of the ethanoic acid
In each case, the base has been diluted - so the salt concentration in (1) is 0.05 M
The cation concentration is the same - 0.1M. So the salt/acid part of the equation is 1/2. Log of this is -0.3. So pKa is pH + 0.3.
I calculate the pKa's to be 4.46, 4.8, 4.52, which doesn't look right. The calculated pH values, based on a pKa for ethanoic acid of 4.76 should be 4.76, 5.76, 3.76, versus the OPs values.
Hmmmm....
It has been 40 years since I learnt this - I may have taught this ~ 20 years ago.....
This is lab data - is it possible that the OP measured his solutions incorrectly (not too likely)? Or that the pH meter wasn't calibrated correctly (quite possible)?