A buffer solution is prepared by adding NaC2H202 to a solution of HC2H2O2 (acetic acid).
HC2H2O2(aq) = H+ (aq) + CH3O2 (aq)
What happens if HCl is added?

Shift to reactants(left)

A buffer solution is a solution that can maintain a good pH value due to the addition of a little acid or a little base or dilution.

The buffer solution can be acidic or basic

Acid buffer solutions consist of weak acids and their salts.  

A buffer solution of NaC2H202    and HC2H2O2 (acetic acid) is included in the acid buffer

So :

a slight addition of acid (H⁺) will be balanced by the conjugate base

the addition of a small base (OH⁻) will be balanced by the weak acid

With the addition of acid (H +), the equilibrium will shift to the left, in the formation of CH3COOH. The added acid will be neutralized by the conjugate base component (CH3COO−).

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.