Hydrogen sulfide (HS) is a weak electrolyte. It is partially ionized when in aqueous solution, therefore there exists an equilibrium between un-ionized molecules and constituent ions in an aqueous medium as follows:
:HS       H  +  HS
By applying the law of mass action, we have
Hydrochloric acid (HCl) is a strong electrolyte, which nearly completely ionizes as
:HCl      →  H   +    Cl
If HCl is added to the HS solution, H a common ion and creates a common ion effect. Due to the increase in concentration of H ions from the added HCl, the equilibrium of the dissociation of HS shifts to the left and keeps the value of  K  constant. Thus the dissociation of HS decreases, the concentration of un-ionized HS increases, and as a result, the concentration of sulfide ions decreases.