Question

An aqueous solution of phosphoric acid, H$_3$PO$_4$ being titrated has molarity equal to 0.25 M. Which of the following could be normality of this solution?

• A. 0.25 N
• B. 0.50 N
• C. 0.75 N
• D. All of these

Hint:

For polybasic acids, normality changes with number of H$^+$ neutralized.

Answer 1

The Correct Option is D

Concept: Normality depends on the number of replaceable H$^+$ ions. For polyprotic acids: \[ N = n \times M \]

Step 1: Identify nature of acid: H$_3$PO$_4$ is triprotic (3 replaceable H$^+$).

Step 2: Different stages of neutralization:

• First stage: $n=1 \Rightarrow N = 0.25$
• Second stage: $n=2 \Rightarrow N = 0.50$
• Third stage: $n=3 \Rightarrow N = 0.75$

Step 3: All these normalities are possible depending on extent of neutralization.
Conclusion:
All given values are possible.