Question

The active mass of $64 gm$ of HI in a two liter flask would be:
A. 2
B. 1
C. 5
D. 0.25

Answer 1

Hint: The term active mass refers to the concentration of a reacting substance usually in terms of moles per liter. Moreover, it refers to the mass of a substance that is actually reacting. It is basically the concentration of the portion of a dissolved electrolyte that is dissociated into ions and hence is capable of carrying the electric current.

Formula used:
Active mass $ = \dfrac{{Number\,of\,moles}}{{Volume\,in\,litres}}$
Further, number of moles$ = \dfrac{{Given\,mass}}{{Molar\,mass}}$

Complete step by step answer:
The law of mass action states that the frequency of any chemical reaction is proportional to the sum of the masses of the reacting materials, each mass being elevated to a power equal to the coefficient in the chemical equation.
Further, the active mass is the concentration of a reacting substance in terms of moles per litre.
Now, we have been given the mass of HI i.e. $64g$ and its volume, which is $2L$. Lets calculate the active mass.
So, firstly we will calculate the number of moles according to the given formula.
Therefore, number of moles $ = \dfrac{{given\,mass}}{{molar\,mass}}$
$ = \dfrac{{64}}{{128}}$ (Molar mass of HI is $128g$)
= 0.5
Now, we’ll calculate the active mass according to the given formula
So, active mass $ = \dfrac{{number\,of\,moles}}{{volume\,in\,litres}}$
$ = \dfrac{{0.5}}{2}$
= 0.25

Hence, option D is correct.

Note: A peculiarity of the theoretical progress was that, at first, the law of mass action was known as the kinetic law of mass action. Furthermore, the empirically grounded principle that the frequencies of chemical reactions is proportional to the number of active masses of reactants and is the kinetic law of mass action leads to the dynamic interpretation of chemical equilibrium, and hence to the law of mass action on equilibrium.