Question

Why active mass of pure solid is taken as one whereas other is not?

Answer 1

For solids or liquids, as mass decreases the volume also decreases by a proportional amount. 1000L water and 10mL water both have molar densities of 55.5 mol/L. So molar density variations are minimum and hence the variations in molar activity is minimum. So the activity coefficient is taken as unity. Edit: The same cannot be said for gases as gases donot have a fixed volume. So for example even if we start with 10L of O2 for combustion of carbon in a closed vessel. The number of moles of O2 would decrease as the reaction proceeds but the volume of O2 would still be 10L. Hence the molar density of O2 would decrease and so would the activity.