Question

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

Answer 1

The concentration of a substance in a reaction represented generally in moles per litre. Active mass is concentration in $mol l^{-1}$ .In simple terms active mass can be defined as g mol per litre.

Formula used:
Active mass $= {\text{ }}{\mathbf{w}}/{\mathbf{(mV)}}\left( {\mathbf{l}} \right)$

Complete step by step solution:
Active mass of 64g of HI in a two litre flask can be measured by:
As we know,
H = $1$ g/mol
I = $127$ g/mol
Active mass = $64/(128 \times 2)$
Where molecular weight is $128$ and volume is $2$ litre.
By calculating, active mass comes out to be $0.25$.

Hence, Option D is correct.

Additional information:
Active mass is usually defined as the molar concentration which is no. of Gram-moles per 1 litre. Molar concentration is directly related to density. Since solid or liquid density always stay the same, so the active mass is taken as one. The solid or liquid density reactant does not vary over the time period of a reaction.
Active mass, or mass of reaction, or as it is more normally known as chemical activity, is the total amount of compound substance that is reactive where the reaction is taking place. The unit is $mol/m^3$ in the SI, normally because space is 3-D, if a reaction occurs within a volume.
The molar concentration of a compound substance occurring in a reaction, the rate of reaction is proportional.

Note: This is a typical formula based question, total molecular mass needed to be calculated firstly, then with the help of molecular mass, volume and amount of weight active mass can be found just by putting values in the formula given above.