which pairs will form a buffer solution?

A buffer is a solution containing a weak acid and its conjugate base salt or a weak base and its conjugate acid salt.

For example,

(1) NaI + HI.

HI is a weak acid

HI --> H + I-

NaI is a salt

NaI --> Na+ + I-

Notice they both dissociate to form iodide anions.

Now consider what happens when you add acid or base

(a) add acid

the addition of acid will drive the formation of HI from the acid-generated protons and the I- anions in solution. This will happen until you start running out of I- anions to "abosorb" the acid protons, at which point the acid will start to increase pH instead. Thus, the presence of the salt NaI provides lots of I- to protect against pH changes on addition of acid.

(b) add base

if you add a base, the protons from HI will neutralize the base and you'll just get water and I-, until you run out of HI. This means the weak acid protects against pH shock by bases.

This equilibrium works as long as the concentration of weak acid to conjugate base salt (or weak base to conjugate acid salt) is within a factor of 10. After that, the buffering capacity is pretty much shot (you can verify with the Henderson-Hasselbach equation, pH = pKa + log ([base]/[acid]).

In (1) (2) and (4) you have conjugate acid-base pairs, wheras in (3) the components are a strong acid and a strong base, which will just neutralize each other.

2) Because addition of H+ causes NH3 + H+ ===> NH4+, while NH4+ + OH- ===> NH3 + H2O

3) Because of the above. You have enough HCl + NH3 to make an NH3 + NH4Cl solution.

4) Because CH3COOH + OH- ===> CH3COO- + H2O, while CH3COO- + H+ ===> CH3COOH