Verified answer

a) will certainly become less acidic. The pH will increase. That's because the oxalate ion concentration has increased, so the equilibrium between

oxalic acid <-------> oxalate ion and proton

will have to shift back to the left to re-establish.

So som of the freshly introduced oxalate ions combine with protons and form the oxalic acid molecule.

b) the ammonium ion is capable of releasing protons. If you add it to a neutral or alkaline solution it will release protons to form ammonia.

However, in the presence of a strong acid the effect will hardly be noticeable.

in theory it would reduce the pH by dissociating into NH3 and H+, but the actual change in pH would be very small.

c) there is not even a theoretical reason for any change in pH in this example. Both salts dissociate completely. The acetate ion itsself will cause the solution to be weakly alkaline, but addition of NaCl won't change anything.