Question

Which one does not give a buffer solution?
(A) Ammonia and sodium hydroxide in water
(B) Sodium acetate and acetic acid in water
(C) Ammonia and ammonium chloride in water
(D) Sodium acetate and hydrochloric acid in water

Answer 1

Hint: According to definition of buffer solutions, a mixture of a weak acid and its conjugate base or a mixture of a weak base and its conjugate acid can act as a buffer solution.

Complete step by step answer:
- Buffer solutions are the solutions that resist the change of pH of the solution upon addition of small amounts of acid or base to the solution. So, let’s see what is required to make a buffer solution.
- A mixture of a weak acid and its conjugate acts as a buffer solution. Same way a mixture of a weak base and its conjugate acid also acts as a buffer solution.
- Conjugate acid is the acidic species formed upon dissociation of base and conjugate base is the basic species formed upon dissociation of acid. Now, let’s analyze the given option to check whether they are buffer solutions or not.
A) Ammonia is a weak base and sodium hydroxide is also a strong base. So, according to the definition of buffer solution, this mixture cannot act as a buffer.

B) Acetic acid is a weak acid. Sodium acetate will dissolve in water to produce acetate ions. This reaction can be given as under.
     \[C{{H}_{3}}COONa\to C{{H}_{3}}CO{{O}^{-}}+N{{a}^{+}}\]

Now, acetate ion is a conjugate base of acetic acid as well. So, it is a mixture of a weak acid- acetic acid and its conjugate base-acetate ions. So, it is a buffer solution.

C) Ammonia is a weak base and ammonium chloride will dissociate in water as shown below.
     \[N{{H}_{4}}Cl\to N{{H}_{4}}^{+}+C{{l}^{-}}\]

So, ammonium ions are conjugate acid of ammonia and so we can say that this is a mixture of a weak base and its conjugate acid. So, this is also a buffer solution.

D) HCl is a strong acid but it will react with sodium acetate as shown below.
     \[C{{H}_{3}}COONa+HCl\to C{{H}_{3}}COOH+NaCl\]

Thus, in this way a weak acid acetic acid will get formed upon this reaction. Now, acetate ions are conjugate bases of acetic acid. So, this mixture is also a buffer solution.
So, the correct answer is “Option A”.

Note: Remember that ammonia is not a strong base, only KOH and NaOH are the strong bases. Upon dissociation of acid, the negatively charged ion (anion) is the conjugate base of that acid and upon dissociation of a base, the positively charged ion (cation) is the conjugate acid of that base.