Question

In a buffer solution consisting $Na{H_2}P{O_4}$and $N{a_2}P{O_4}$
(1) $Na{H_2}P{O_4}$ is acid and $N{a_2}HP{O_4}$ is salt
(2) The $pH$ of solution can be calculated using the relation:
$pH = p{K_2} + {\log _{10}}\dfrac{{\left[ {HPO_4^{2 - }} \right]}}{{\left[ {{H_2}PO_4^ - } \right]}}$
(3) The $N{a_2}HP{O_4}$ is acid and $Na{H_2}P{O_4}$ is salt
(4) The $pH$ cannot be calculated.
(A) 1,2 and 3 correct
(B) 1 and 2 are correct
(C) 2 and 4 are correct
(D) 1 and 3 are correct

Answer 1

Hint: Buffer solution: $Na{H_2}P{O_4}$and $N{a_2}HP{O_4}$ are an acid / base conjugate pair. They will make excellent buffers.
Buffer solution is aqueous solution of a weak acid and its conjugate base or vise-versa.
Buffer solutions resist $pH$ change upon the addition of an acidic or basic component.

Complete step by step answer:
Let us discuss buffer solution by given example i.e., buffer solution containing $Na{H_2}P{O_4}$ and $N{a_2}HP{O_4}.$
Both have acidic hydrogen but $Na{H_2}P{O_4}$has two acidic hydrogen and $N{a_2}HP{O_4}$ has one acidic hydrogen.
Therefore,$Na{H_2}P{O_4}$ acts as an Acid
$N{a_2}HP{O_4}$ acts as a salt is solution
The salt is formed by weak acid $Na{H_2}P{O_4}$ with strong base $NaOH.$
$Na{H_2}P{O_4} + NaOH \to N{a_2}HP{O_4} + {H_2}O$]
$\therefore $The mixture contains weak acid $Na{H_2}P{O_4}$and salt $N{a_2}HP{O_4}$ with a strong base.
Therefore, the given buffer is an acidic buffer.
$pH$of buffer containing weak acid and salt with strong base is given by.
$pH = PKa + \log \dfrac{{[salt]}}{{[Acid]}}$
In given buffer solution $pH$of solution is
$pH = PKa + \log \dfrac{{[HPO_4^{2 - }]}}{{[{H_2}PO_4^ - ]}}$
Therefore, in given question statement (1) and (2) are correct

So, the correct answer is “Option B”.

Additional Information:
Buffer action of the above solution can be explained as.
If strong acid like $HCl$ is added to the buffer solution the additional ${H^ + }$combines with weak acid ions in the solution to produce undissipated weak acid.
Therefore, $pH$ of the solution remains unchanged.
If solution is buffered, addition of acid / base changes the $pH$ only by a small amount.

Note:
Buffer solution is able to neutralize a small amount of adder acid or base thus it maintains the $pH$ of solution stable.
This is required for processes or reactions which require specific and stable $pH$ range.