Question

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

Answer 1

Active mass implies to the amount of mass which is taking part in a reaction i.e. it is the effective concentration of a substance. Its other names are reactive mass or chemical activity. Active masses are dimensionless quantities but for our purposes we generally take them with dimensions of molarity, partial pressures etc.

Complete step by step answer:
As already mentioned in the hint, active mass is simply that concentration of the substance which is available in the reaction and is actively taking part to complete the particular reaction.
The active mass of a substance is similar to the concentration of the substance in the given solution. Since, it is similar to concentration; it is mathematically expressed as similar to that of the concentration. It can be represented as:
Active mass = $\dfrac{n}{V}$ … (i)
Where, $n =$ number of moles of the substance = $\dfrac{w}{{{M_w}}}$
Here, $w =$ given weight = $64g$
${M_w} =$ molecular weight of $HI = 128g$
$V = 2l$
Thus, substituting these values in equation (i), we have:
Active mass = $\dfrac{n}{V} = \dfrac{{\dfrac{{64}}{{128}}}}{2} = 0.25$
Thus, the correct option is D. $0.25$ .

Note:
The units of activity are nominal, rather than real, because chemical activity is formally defined as the ratio of the actual chemical activity of a substance to its chemical activity under some defined standard conditions, and ratios have no units because the units divide out. Or simply it's the ratio of concentration at required conditions to standard condition. The difference between activity and other measures of composition arises because molecules in non-ideal gases or solutions interact with each other, either to attract or to repel each other. The activity of an ion is particularly influenced by its surroundings.