Question

Which of the following mixtures will have the lowest $pH$ at $298{\text{ }}K$ ?
(1) $10{\text{ml 0}}{\text{.05N C}}{{\text{H}}_3}{\text{COOH + 5ml 0}}{\text{.1N N}}{{\text{H}}_4}{\text{OH}}$
(2) ${\text{5ml 0}}{\text{.1N N}}{{\text{H}}_4}{\text{Cl + 5ml 0}}{\text{.2N N}}{{\text{H}}_4}{\text{OH}}$
(3) ${\text{5ml 0}}{\text{.01N C}}{{\text{H}}_3}{\text{COOH + 10ml 0}}{\text{.05N C}}{{\text{H}}_3}{\text{COONa}}$
(4) ${\text{5ml 0}}{\text{.1N C}}{{\text{H}}_3}{\text{COOH + 5ml 0}}{\text{.2N N}}{{\text{H}}_4}{\text{OH}}$

Answer 1

Hint : The normality of an acid is defined as the concentration of the acid times the number of acidic protons per molecule. The molarity of an acid is inversely proportional to the $pH$ value. It means if we increase the molarity of an acid, then its $pH$ value decreases.

Complete Step By Step Answer:
The gram equivalent weight of the given solute per liter of solution is called normality of the solution. The gram equivalent weight is the measure of reactive capacity of a given chemical species like ions, molecules, etc. Normality is generally used to express the concentration of hydronium ions or hydroxide ions present in a solution. The product obtained in the first option is $C{H_3}COON{H_4}$ . The $pKa$ value of acetic acid and the $pKb$ value of $N{H_4}OH$ are the same and are equal to seven as they neutralize completely. Option two is an example of a basic buffer. So, the $pH$ will be more than $7$ . Option three is an example of an acidic buffer. So, the $pH$ is less than $7$ . The last option gives an aqueous solution of $C{H_3}COONa$ which hydrolyses to form a basic solution.
Therefore, option 3 is the correct answer.

Note :
Remember that n-factor an acid is the basicity of that acid. It helps in finding the gram equivalent of a compound. In a reaction, the total number of ${H^ + }$ ions gives the n-factor. The equivalent weight of an acid is the mass of acid which is needed to generate one mole of hydronium ions in an aqueous solution.