The correct option is A Molar concentration of weak base is more than the molar concentration of strong acid i.e [NH4OH]>[HCl]
Weak Base (WB) and Strong Acid (SA)
Basic buffer (pH >7) : It is the mixture solution of a weak base and a salt of this weak base with a strong acid.
Consider the combination of NH4OH and HCl
Condition-1 where[WB]>[SA]
NH4OH(aq)+HCl(aq)→NH4Cl(aq)+H2O(l)
4 3 − −
1 0 3 3
As 1 mol of NH4OH is left,
The reaction here is NH4OH(aq)⇌NH+4(aq)+OH−(aq)
1(1−α) α+3 1α
≈3
α is too small as compared to the concentrations of [NH+4] and [NH4OH]
We get Kb=[NH+4][OH−][NH4OH]
[OH−]=Kb[NH4OH][NH+4]
∴ By addition of a small amount of H+ ions equilibrium will move in forward direction by the same amount and same number of OH− ions are produced. Hence there is no significant change in the pH is observed.
This acts as a buffer solution.
Condition-2 where[WB]=[SA]
NH4OH(aq)+HCl(aq)→NH4Cl(aq)+H2O(l)
3 3 − −
0 0 3 3
(Hydrolysis occurs here as there is complete consumption of acid and base as a result salt is only left in solution. This is not a buffer)
Condition-3 where [WB]<[SA]
NH4OH(aq)+HCl(aq)→NH4Cl(aq)+H2O(l)
3 4 − −
0 1 3 3
Hydrolysis of NH4Cl can be neglected as degree of hydrolysis was already small and common ion effect due to H+ coming from HCl will further decrease it.
So, The [H+]total can be assumed to be from HCl only andpH can be directly found from this [H+].
This is not buffer solution