Question

Using a pH 10 buffer solution of aqueous ammonia and aqueous ammonium chloride, which chemical species is the weak base? Which is the conjugate acid? How do you know?

Answer 1

H20(l) + NH3 (aq) <----> NH4+ (aq) + OH- (aq)

Here water is acting as an acid, donating the H+; OH- is called the conjugate base of H2O.

Similarly, NH4+ is called conjugate acid of the base NH3, because NH4+ can donate to reform NH3.

Here weak base is NH3 and its conjugate acid is NH4+

The equilibrium constant is known as base ionisation constant Kb

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

If Kb is much greater than 1, the base reacts nearly completely with water and so is said to be a
strong base. If Kb is much less than 1, the base reacts hardly at all with water and so is said to be a
weak base.

from the equilibrium constant value, we can tell that it is weak base or not.
Kb for ammonia is 1.8 × 10-5, which is very small, so ammonia is a weak base.
The ammonium ion, NH4+, is the conjugate acid of ammonia, NH3.