[Carnivorouss]

I know that NH4+ is a weak acid, and a conjugate acid of NH3. I have had an argument with my teacher why it is acidic. I think it is a weak acid because of the hydrolysis equation: NH4+ + H2O <->NH3 +H3O+. This reaction produces H3O+ ions, thus, making the solution acidic.

HOWEVER, my teacher is saying that it is because of the the shift in equilibrium to the right of the autoionisation of water: H2O -> H+ + OH-. He is saying that NH4+ + OH- -> NH3 + H2O. The decrease in the [OH-] means that according to La Chatelier's Principle the water ionisation equilibrium shifts to the right producing H+ ions, thus, making the solution acidic.

Both mine and my teacher's way seem logical. However, i think the NH4+ is reacting with water MOLECULES via hydrolysis to produce H+, while my teacher is saying that NH4+ is reacting with OH- present in water shifting the equilibrium to the right, thus, producing H+ ions.

Who is right and why? I really need help because the teacher took marks off me in the test for using my explanation rather than his.

[Borek]

I see nothing wrong with your answer, apart from the fact yours is not a hydrolysis equation, but just a dissociation.

Because of the stoichiometry water ionization can't produce more H⁺ than OH^-. pH of the 0.1 M ammonium chloride solution is around 4.6. Solution contains around 4.2×10^-10 OH^-, and only that much H⁺ was produced from the water autoionization.

Where does the rest of the 2.4×10^-4 M H⁺ come from?

[AWK]

The problem lays probably on the level of theory of acids and bases you should use.
Im water solution of ammonium salt (obtained from strong acid) two basic equilibria exist (hydrolysis af ammonium salt and autodissociation of water - according to Arrhenius theory, or protolysis of water by ammonium ion and autoprotolysis of water - according to Bronsted-Lowry theory).
Calculation of concentrations or pH in both theories, exact or approximated, are practically the same.
These equilibria are:
NH₄⁺ + H₂O = NH₃ + H₃O⁺ (spectator ion omitted)
and
2H₂O = H₃O⁺  + OH^- in Bronsted-Lowry theory or H₂O = H⁺ + OH^- in Arrhenius theory.
Moreover, you may expect reactions:
NH₄⁺ + OH^- = NH₃ + H₂O
and
H₃O⁺ + OH^- = 2H₂O (B-L theory) or H⁺ + OH^- = H₂O (Arrh. theory).
Equilibrium constants for these reactions can be derived from the previous ones.
In , say, 0.1 M NH₄Cl solution (I follow Borek) you have 0,1 M NH₄⁺ and ~54 M H₂O and 10^-7 M for both concentration of H₃O⁺ (H⁺) and OH^- at the beginning.
Protolytic reactions are very fast. Then firstly proceed reactions with highest concentration of reagents (and with the highest equilibrium constant - in this case), ie protolysis of water by ammonium ions (B-L theory) or hydrolysis of ammonium ions (Arrh. theory), then reaction of ammonium ions with hydroxide ions. Since the first reaction increase the concentration H₃O⁺ (H⁺)  then the next expected reaction will be the reaction of this ions with hydroxide ions. Because of lower value of constants for last two reactions and very low concentration of the hydroxide ions these reaction only minimally diminishes the concentration of H₃O⁺ (H⁺) ion obtained in the first reaction (though substantially change the concentration of OH^- ions) and usually for calculation of pH the later two reactions are neglected.