Question

assertion: degree of hydrolysis and pH of a salt say $N{H_4}CN$ is independent of concentration of $N{H_4}CN$ .
Reason: the solution of $N{H_4}CN$ water has a pH slightly greater than 7.
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct but reason is not the correct explanation for assertion
C. Assertion is correct but reason is incorrect
D. Assertion is correct but reason is incorrect

Answer 1

Hint: pH of a solution is the negative of the base $10$ logarithm of hydronium ion concentration in a solution. The concentration of hydronium ions in an aqueous solution can provide information about the acidic, basic or neutral nature of a solution. For an acidic solution, pH is less than $7$ and for a basic solution, pH is greater than $7$. At room temperature, the pH of neutral water is equal to $7$.We know the salt are formed when acid and base combine with each other . salts consist of cationic and anionic parts to make a product neutral compound .

Complete step by step answer:
As we know the definition of salt , they form when acid and base react together and make water as a side product . For example formation of sodium hydroxide , when hydrochloric acid and sodium hydroxide react together they come with sodium chloride as a product with water as a side product . reaction may be written as - $NaOH + HCl \to NaCl + {H_2}O$
The acidity or basicity of salt depends upon the reactants , if salt is formed when a strong acid reacts with a weak base , thus the salt formed is acidic in nature and vice versa in case of strong base .
pH scale is used to measure or detect the compound is acidic or basic in nature , if the pH scale showing at 7 salt is neutral if lower than 7 then the salt having acidic characteristics and above 7 which means salts having basic characteristics .
now coming t solution part of above mention question ; ammonium cyanide is formed when ammonium hydroxide react with hydrogen cyanide the reaction may be written as - $N{H_4}OH + HCN \to N{H_4}CN + {H_2}O$
And we know that salt hydrolysis is a reverse of neutralization, then
$N{H_4}OH \rightleftharpoons N{H_4}^ + + O{H^ - }$
$HCN \rightleftharpoons {H^ + } + C{N^ - }$
pH can be given for weak acid and weak base = $pH = 7 + \dfrac{1}{2}\left( {p{k_a} + p{k_b}} \right)$
which is independent of concentration , hence assertion is correct .
comes to the reason part , $N{H_4}OH$ Is stronger base than $HCN$ which means the concentration of hydroxide ion in solution is more which makes solution slightly basic , hence the reason is also correct
but reason is not a valid reason for assertion .

So, the correct answer is Option B.

Note: Salts completely hydrolyze in water to give ions but weak acid and base are not completely hydrolyzed in water, they partially hydrolyzed in water.
To determine the strength of acids and bases, we use a universal indicator i.e. pH which shows a different concentration of hydronium ion concentration in a solution.
The pH scale ranges from $0 - 14$.
Glass electrode, pH metre or colour changing indicator may be used to measure the pH of an aqueous solution