If you calculate it with the help of formula i.e mol/vol(lit) the answer will be two.But this is not correct as the active mass of solids is always taken as 1 because density and molecular mass of solid are constant. So in the conclusion the answer will be 1.
						

							This question is mainly based on your concepts....... If you will apply your basic formula for active mass, i.e. mol/vol(litre), you will get 1/2. but That is wrong....... Correct explanation of This question is as follow..     The active mass of solids and pure liquids is a constant quantity(unity) because it is an intensive property i.e. number of molecules present per unit volume do not change because density and molecular weight of solids and pure liquids are constant.. But it doesnt apply for gaseous sunstances because for them no. of gaseous molecules present per unit volume change with change in volume of vessel.......      since Nacl is a solid. That is why active mass of solid i. e. Nacl is 1.
						

							For pure liquids and solids active mass is 1. This is mainly because of the fact that it remains constant or it`s change is very very small