Question

A 0.0884 M solution of a weak base has a pH of 11.79. What is the identity of the weak base? Weak Base Kb Ethylamine (CH3CH2NH2) 4.7 X 10–4 Hydrazine (N2H4) 1.7 X 10–6 Hydroxylamine (NH2OH) 1.1 X 10–8 Pyridine (C5H5N) 1.4 X 10–9 Aniline (C6H5NH2) 4.2 X 10–10

Answer 1

pH of the solution of the weak base = 11.79

We know that pH and pOH are related as

pH + pOH = 14

Hence, pOH of the solution can be calculated as

pOH = 14 - pH = 14 - 11.79 = 2.21

pOH is the negative logarithm of the OH- concentration. Hence, the equilibrium OH- concentration of the solution can be calculated as

Let the identity for the acid be A.

Hence, the reaction it undergoes in water is

Hence, the equilibrium constant Kb for the base A is expressed  as

where the concentrations are their equilibrium values.

Note that H2O is the pure solvent and does not appear on the equilibrium constant expression.

Also note that the amount of the conjugate acid AH+ produced must be stoichiometrically same as that of OH-.

Hence, we can write

Hence, given that the initial concentration of the base A is 0.0884 M, we can create the following ICE table

Initial, M	0.0884	0	0
Change, M	-x	+x	+x
Equilibrium, M	0.0884-x	x	x

Note that we already know the value of x from the pH data by calculating the equilibrium OH- concentration.

Hence, the equilibrium constant expression can be written as

The values of Kb for the weak bases given are

Base	Kb
Ethylamine
Hydrazine
Hydroxylamine
Pyridine
Aniline

From the table, our Kb value matches closely with that of Ethylamine, while the other Kb values are orders of magnitude smaller that the Kb of ethylamine.
Hence, our weak base must be ethylamine.