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−).

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.