Eq(21.52) is valid for ideal gases.For real gases, the pressures get replaced by effective pressures or fugacities f (which are defined by RT ln (f₂ /P) = _P^P₂VdP where f₂ is the fugacity at pressure P₂ . If the gas is ideal, f₂= P₂) and for liquids and solids, they get replaced by activities a. The activity a is defined as relative fugacity, a = f / f _o, where f _o is the activity in a standard state. For example, if a solution is in equilibrium with its vapour, the chemical potentials or the fugacities of each component is the same in the liquid phase and the vapour phase. If the vapour pressure of say, component A is 400 mm Hg and the vapour pressure of its pure component (i.e., pure A) is 500 mm Hg, then the activity of the component A in the solution is 400/500 = 0.80. Here, we have assumed that the vapour behaves ideally and therefore, fugacities can be replaced by pressures. Fugacities and activities are central to all equilibrium constant estimations. Some of these aspects will be taken up in the chapter on electrochemical cells (Chapter 22).