Question

An aqueous solution of phosphoric acid, $H_{3}P{O}_4$ being titrated has a molarity equal to 0.25M. Which of the following could be normality of this solution?
A. 0.25N
B. 0.50N
C. 0.75N
D. None of these

Answer 1

Hint: To solve this question you should have basic knowledge of the term Normality, molarity, n-factor and equivalents weight.Phosphoric acid releases 3 hydronium ions when dissolved in the solvent. Thus, its equivalent weight is one-third of its molecular weight.

Complete step by step solution:
Normality is defined as the number of gram or mole equivalents of solute present in one liter of a solution. It is denoted by the letter N. Equivalent weight is known as the mass of one equivalent, i.e. the mass of a substance taken under consideration which will combine with or displace a definite quantity of another substance in a chemical reaction. For an element the equivalent weight is nothing but the mass which combines with or displaces 1.008 gram of hydrogen or 8.0 gram of oxygen or 35.5 gram of chlorine. It is also known as gram equivalents.
Molarity is defined as the number of moles of solute dissolved in one liter of the solution. It is denoted by the letter M.
N-factor for acids, n-factor is defined as the number of $H^{+}$ ions replaced by 1 mole of acid in a reaction. For bases, it is defined as the number of $O{H}^{-}$ ions replaced by 1 mole of base in a reaction.
$Normality=(N-factor)\times (molarity)$
1 mol of $H_{3}P{O}_4$ gives 3 moles of $H^{+}$ ions.
In Phosphoric acid $H_{3}P{O}_4$ number of $H^{+}$ ions it produces on dissociation per molecule of acid is 3. Hence the n-factor for phosphoric acid $H_{3}P{O}_4$ is 3.
Molarity is given in question 0.25M.
Hence, Normality = $3\times 0.25=0.75N$
Hence, option C is correct.

Note:
Don’t get confused between molarity and molality. Molarity is defined as the number of moles of solute dissolved in one liter of the solution whereas molality is defined as the number of moles of the solute per kilogram of the solvent.
N-factor is not the measure of basicity ; it is instead the number of moles of $H^{+}$ atoms present in one mole of acid which are replaceable.
N-factor is not the measure of acidity; it is instead the number of moles of $O{H}^{-}$ atoms present in one mole of base which are replaceable atoms.