Question

Calculate the equivalent mass of $HCl$ .
A. $31$
B. $35.5$
C. $36.5$
D. $18.2$

Answer 1

Equivalent mass or the gram equivalent is the mass of one equivalent, i.e. the mass of a given substance which will combine with or displace a fixed quantity of another substance. The equivalent mass of an element is the mass which combines with or displaces $1.008\;gram\;$ of hydrogen or $8.0{\text{ }}grams$ of oxygen or $35.5{\text{ }}grams$ of chlorine.

Complete step by step answer:

In order to calculate the equivalent mass of any compound, we need to determine the basic two constituents of a compound that is under observation. These two basic utilities are the molar mass and the n- factor or the number of electrons transferred during the formation of the chemical compound. The molar mass is defined as the mass of one mole of any substance and in order to determine the molar mass of any substance, one must determine the atomic mass of the constituent elements. In the case of formation of hydrochloric acid, the following reaction takes place.
$H \to {H^ + } + {e^ - }$
$Cl + {e^ - } \to C{l^ - }$
Hence, combining both the reactions, we find that:
${H^ + } + C{l^ - } \to HCl$
From the above reactions, it is clearly visible that the n- factor is equal to one.
Now, we need to determine the relation between equivalent mass, molar mass and n- factor. The relation is as follows:
$E = \dfrac{M}{n}$
Where, $E =$ equivalent mass of the substance
$M =$ Molar mass of the substance
$n =$ n- factor
The molar mass of $HCl = 1 + 35.5 = 36.5$
Hence, the equivalent mass of $HCl = \dfrac{M}{n} = \dfrac{{36.5}}{1} = 36.5$
Thus, the correct option is C. $36.5$.

Note:
The use of equivalent mass has been superseded by molar mass as it is easier to use and there is less ambiguity while using it. Equivalent mass has various applications such as in general chemistry, gravimetric analysis, volumetric analysis, in polymer chemistry, etc.