Question

$0.4g$ of polybasic acid, ${H_n}A$ (all the hydrogens are acidic) requires $0.5g$ of $NaOH$ for complete neutralisation.
The number of replaceable hydrogen atoms and the molecular weight of A would be respectively:
(Molecular weight of acid $= 96g/mol$ )
A. $2,94$
B. $1,93$
C. $3,93$
D. $4,92$

Answer 1

To solve these kinds of questions the most used and best way is the molar approach. It provides us the best option to compare the values directly without any interconversion.

Complete step by step answer: The aforementioned reaction is an example of neutralization reaction. In a neutralisation reaction the equivalents of the two species neutralising each other is always the same. The moles if taken as $molarity$ $\times$ $volume(ml)$ $\times$ $n-factor.$ is known as $milliequivalents$
Hence, the milliequivalents should be same:
Milliequivalents of acid $=$ milliequivalents of base
$\dfrac{{0.4}}{{\dfrac{{Mw}}{n}}}*1000 = \dfrac{{0.5}}{{\dfrac{{40}}{1}}}*1000 \\\ n = 3 \\\$
Where $n$ is the $n-factor$ or number of replaceable hydrogen atoms in this case.
Therefore the formula of the acid will be: ${H_3}A$
And,
${H_3}A = 96 \\\ A = 96 - 3 \\\ A = 93 \\\$
So the correct option is: C

Note: Most important thing here is to write the balanced chemical equivalents as it is the initial and generally most careless state. Apart from that it is necessary to find the correct formulations. It is most common mistake of the wrong milliequivalents that lead to various wrong answers despite correct execution.