Why is a mixture of HCl and NaCl function poorly as a buffer?

Question

Is it because they are salts?

Answer 1

It's because HCl is a strong acid, and 100% ionised in solution. The acid used in a buffer has to be a weak acid.

Answer 2

To make a good buffer you need the salt of a strong base and a weak acid along with a weak acid. In the example given HCl and NaCl you have the salt of a strong base and strong acid along with a strong acid. There are no species to absorb or convert strong acids into weak ones or strong bases into weak ones.
A good buffer would be able to absorb acids (H+ ions) or bases (OH-) ions. Bicarbonate is such a buffer. It is made of carbonic acid (H2CO3) which is a weak acid and sodium or potassium bicarbonate (NaHCO3)(K2CO3)

If a strong acid is added such as HCl then this reaction occurs
HCl + NaHCO3 ==> NaCl + H2CO3 (H2O + CO2)

The strong acid is converted into a weak one (carbonic acid).
If a strong base is added such as NaOH then this reaction occurs
NaOH + H2CO3 ==> NaHCO3 + H2O

The strong base is converted into a weak one (sodium bicarbonate)
Notice that the species that are formed are part of the buffer system.
The strong acid is