the term active mass is defined as the molar concentration oer unit volume (number of moles per dm³ of a subatance

 

consider the example

 

in a reaction  vessel of one cubic metre volume, hydrogen gas reacts with gaseous iodine to form hydrogen iodide gas at 300K . at equilibrium 2.5 moles of  hydrogen 6.0 moles of iodine, 25 moles of HI were found to be present.

 

   the equation is                                    H_²        +       I₂     =      2HI

no of moles  at equilibrium                        2.5             6              25

 

since volume is 1 m³  , active mass           2.5/1          6/1             25/1