Question

A buffer solution can be prepared from the mixture of:
(i) Sodium acetate and acetic acid in water.
(ii) Sodium acetate and hydrochloric acid in water.
(iii) Ammonia and ammonium chloride in water.
(iv) Ammonia and sodium hydroxide in water.
[A] (i) , (ii)
[B] (ii) , (iii)
[C] (iii) , (iv)
[D] (i) , (iii)

Answer 1

Hint: Buffer solutions are of two types- Acidic buffer and Basic buffer. Acidic buffer consists of a weak acid and its salt, along with a strong base. Basic buffer is prepared by mixing a weak base with its conjugate salt along with a strong acid.

Complete step by step answer:
As we know, a buffer solution is used to maintain the pH of a solution at a constant value during titration when a small amount of acid or base is added to the solution.

a.) Firstly, Let us consider option [A] - An acidic buffer solution can be made by mixing a weak acid with its conjugate salt with a strong base, like sodium acetate and acetic acid in water.
As we know, acetic acid is a weak acid. Its conjugate base will be-

$C{H}_{3}COOH-H^{+}\to C{H}_{3}CO{O}^{-}$

To form the conjugate salt, it reacts with the strong base NaOH forming sodium acetate I.e. a salt of a strong base. Therefore, it can be used to make a buffer solution.

b.) In the option [B], we have sodium acetate with hydrochloric acid and water.
Hydrochloric acid can neutralise the sodium acetate partly to give acetic acid. So, the reaction will have both sodium acetate and acetic acid which can act as a buffer. But if the HCl is present in larger quantities, it will undergo complete dissociation forming sodium chloride and acetic acid, which will not be a buffer solution.

c.) In option [C], we have ammonia and ammonium chloride in water which can be used to form a buffer but it will be a basic buffer solution. As ammonia is a weak base and ammonium chloride its salt which can be formed by a strong acid-

$N{H}_3+HCl\to N{H}_{4}Cl$

Therefore, we can use it to make a basic buffer solution.

d.) And lastly in option [D] we have ammonia and sodium hydroxide. Here, we have no salt present. So we cannot use it for the formation of a buffer solution.
Therefore, it is clear that we can use a mixture of sodium acetate and acetic acid and a mixture of ammonia and ammonium chloride in water to form the required buffer solution.

Therefore, the correct option is [D] (i), (iii).

NOTE: It is important to note here that one mixture gives us an acidic buffer and the other gives us a basic buffer. pH of acidic buffers is below 7 and pH of basic buffers is greater than 7.