Question

Active mass is defined as?
A- Number of g equivalents per unit volume.
B- Number of g mol per liter
C- Amount of substance in gram per unit volume
D- Number of g moles in 100 liters

Answer 1

Hint: Active mass is not a measure of mass in the solution. It is the activity of a substance in the solution which is the measure of concentration.

Complete step by step solution:
We should take the active mass of a substance as activity of a substance in the given solution available for reaction. The activity of a substance in a solution refers to the number of moles available for reaction in a unit volume of solution. So concentration is also a measure of the activity of a substance in solution. And generally, the number of moles of substance present in the solution will completely involve a reaction. So the number of moles present can be equated to the number of moles involved in a reaction. Active will be equal to molarity of substance. Molarity of a substance is the number of gram moles per litre of solution.
Therefore, option B is correct.

Note: We may think active mass as a measure of mass present per given amount of volume but this is not true. We also should not confuse between gram moles and gram equivalents. Active mass is not always equal to concentration. For some substances which are present in the solution but only some part of them involve reaction concentration is not equal to active mass.