The degree of dissociation in gases is denoted by the symbol , where  refers to the percentage of gas molecules which dissociate. Various relationships between  and  exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide () dissociating to nitrogen dioxide () will be taken.
If the initial concentration of dinitrogen tetroxide is 1 mole per litre, this will decrease by  at equilibrium giving, by stoichiometry,  moles of . The equilibrium constant (in terms of pressure) is given by the equation
where  represents the partial pressure. Hence, through the definition of partial pressure and using  to represent the total pressure and  to represent the mole fraction;
The total number of moles at equilibrium is , which is equivalent to . Thus, substituting the mole fractions with actual values in term of  and simplifying;
This equation is in accordance with Le Chatelier's principle.  will remain constant with temperature. The addition of pressure to the system will increase the value of , so  must decrease to keep  constant. In fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure is proportional to number of moles) hence decreasing the extent of dissociation .