An aqueous solution of ammonia acts as a weak base because it can accept a proton (H+) from water. When ammonia dissolves in water, it undergoes a reaction with water to form ammonium ions (NH4+) and hydroxide ions (OH-). The equilibrium constant for this reaction is small, which means that only a small amount of ammonia is converted to ammonium ions and hydroxide ions. However, the small amount of hydroxide ions that are produced are enough to make the solution basic.

The reaction between ammonia and water can be represented by the following equation:

NH3 + H2O ⇌ NH4+ + OH-

The equilibrium constant for this reaction is:

Kb = [NH4+][OH-]/[NH3]

At 25°C, Kb = 1.8 × 10^-5. This means that for every mole of ammonia that dissolves in water, only 1.8 × 10^-5 moles of ammonium ions and hydroxide ions are produced.

The small amount of hydroxide ions that are produced are enough to make the solution basic. This is because hydroxide ions are strong bases, and they can completely dissociate in water to form hydrogen ions (H+) and hydroxide ions. The hydrogen ions that are produced by the dissociation of hydroxide ions react with water to form hydronium ions (H3O+). Hydronium ions are responsible for the acidity of solutions.

The pH of an aqueous solution of ammonia can be calculated using the following equation:

pH = 14 - pOH

The pOH of an aqueous solution of ammonia can be calculated using the following equation:

pOH = -log[OH-]

At 25°C, the pH of a 0.1 M solution of ammonia is 11.63. This means that the solution is basic.