Question

Given that $20{\text{mL}}$ of $0.1{\text{N}}$ $NaOH$ was required to neutralize a solution of a dibasic acid. Weight of the acid present in the solution is:
[Molecular weight of the acid=126]
A.$0.63{\text{g}}$
B.$63{\text{g}}$
C.$0.126{\text{g}}$
D.$6.3{\text{g}}$

Answer 1

Hint: To solve this question, you must use the concept of equivalents. Equal numbers of equivalents of an acid neutralize equal numbers of equivalents of a base.

Formula used: ${\text{meq}} = {\text{M}} \times {\text{V}} \times x$
Where ${\text{meq}}$ is the number of milliequivalents
M is the molarity of the acid / base solution
V is the volume of the solution
x is the n-factor of the acid/ base used
Molarity of a solution when multiplied by its n factor gives the normality.

Complete step by step solution:
The n factor of a compound is also known as the valence factor and its value depends upon the compound that is being considered. For instance, for an acid, the basicity of the acid is its n factor. Similarly, for a base, its basicity is taken as the n factor.
For $NaOH$, n- factor $ = 1$ and the acid is dibasic, so, its n- factor $ = 2$
Thus, the equivalent mass of the acid can be written as
$E = \dfrac{M}{2} = \dfrac{{126}}{2} = 63$
For a neutralization reaction, we know that
${\text{meq of acid = meq of base}}$
Substituting the given values of the normality and volume of sodium hydroxide solution, we get,
${N_a}{V_a} = 0.1 \times 20 = 2$
$ \Rightarrow \dfrac{w}{E} \times \dfrac{{1000}}{{{V_a}}} \times {V_a} = 2$
Substituting the values:
$ \Rightarrow \dfrac{w}{{53}} \times 1000 = 2$
Thus, $w = \dfrac{{126}}{{1000}}$
$\therefore w = 0.126{\text{g}}$
The weight of acid that is present in the solution is $0.126{\text{g}}$

Thus, the correct answer is C.

Note: Stoichiometry is based upon the very basic laws of chemistry that help to understand it better, namely, the law of conservation of mass, the law of definite proportions (the law of constant composition), the law of reciprocal proportions and the law of multiple proportions.
In general, different chemicals combine in definite ratios in chemical reactions. Since matter can neither be created nor destroyed, thus the amount of each element must be the same throughout the entire reaction.