Expressing the Equilibrium Constant in Terms of Pressure

So far, we have expressed the equilibrium constant only in terms of the concentrations of the reactants and products. For gaseous reactions, the partial pressure of a particular gas is proportional to its concenttation. Therefore, we can also express the equihbtium constant in terms of the partial pressures of the reactants and products. Consider the gaseous reaction  

From this point on, we designate as the equilibrium constant with respect to concentration in molarity. For the reaction just given, is expressed using the law of mass action  

We now designate as the equilibrium constant with respect to partial pressures in atmospheres. The expression for takes the form of the expression for K, except that we use the partial pressure of each gas in place of its concentration. For the SO3 reaction, we write K.g as  

Since the partial pressure of a gas in atmospheres is not the same as its concentration in molarity, the value of for a reaction is not necessarily equal to the value of K. However, as long as the gases are behaving ideally, we can derive a relationship between the two constants. The concentration of an ideal gas A is the number of moles of A (njf) divided by its volume (V) in liters  

From the ideal gas law, we can relate the quantity nx/V to the partial pressure of A as follows  

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