Mixed Solutions.

[Michelle Yu 1L]

I'm confused of why when you add solid sodium acetate to an acetic acid solution, the concentration of H3O+ decreases
and why when HCl is added to a benzoic acid solution, the percentage of benzoic acid that is deprotonanted decreases
and why when solid NH4Cl is added to an ammonia solution, the concentration of OH- decreases.

These are from Chp 13 Question 1.

[Divya Prajapati 1E]

We can answer all of these questions using Le Chatelier's principle, which applies to any aqueous solution in a state of equilibrium. Because we are dealing with weak acid and base solutions, we know that this is a situation of aqueous equilibrium, because both reactants and products will be present at some concentration at all times.

For the acetic acid solution, we have this equilibrium: acetic acid <---> hydronium ion + acetate ion
When you add sodium acetate, which consists of a neutral sodium ion and an acetate ion, you are essentially adding more acetate ions to the right side of the equilibrium. As a result, the equilibrium shifts left to produce more acetic acid (and therefore less hydronium ions).

For the benzoic acid solution, we have: benzoic acid <---> hydronium ion + benzoate ion
When you add HCl, which consists of a neutral chlorine ion and a hydronium ion, you are adding to the right side. The reaction shifts left to produce more benzoic acid. This means less benzoic acid has deprotonated into hydronium and benzoate ions.

For the ammonia solution, we have: ammonia <---> ammonium ion + hydroxide ion
When you add NH4Cl, consisting of an ammonium ion and neutral chlorine ion, you are adding to the right side. The reaction shifts left to produce ammonia, decreasing the concentration of hydroxide ions on the right.

[Chem_Mod]

Sodium acetate is a source of base. Thus, the hydronium ion concentration decreases when added to a solution of acetic acid.

The percent deprotonation of benzoic acid decreases when strong acid is added to solution because the strong acid will protonate any conjugate base thus shifting equilibrium of benzoic acid dissociation back to the acid form (resulting in less having deprotonated).

The NH₄Cl is a source of conjugate acid of ammonia so it neutralizes some of the OH^- bringing down the [OH^-].