Question

why is ammonium acetate better able to buffer the addition of hcl, compared to ammonium chloride and sodium carbonate

Answer 1

The ammonium acetate buffer:

NH4+ ammonium

CH3COO- acetate

will be able to buffer addition of HCL

since

HCl --> H+ + Cl-

Those H+ in free solution will form:

H+ + CH3COO- --> CH3COOH(aq)

so no more H+ is free, H does not change drastically

Q2.

compare with

NH4Cl and Na2CO3

this will happen:

NH4+ and CO3-2

when H+ is added...

NH4+ can't accept more H+ (i.e. NH5+ won't form)

also

CO3-2 + H+ --> HCO3- but the pH is pretty high at this value , i.e. pH = 10-11

which is not accepted for acidic buffers

therefore,

ammonium acetate buffer is much better since it can accept H+ and OH- ions in the same manner, since pKa = pKb = 4.75