The equilibrium expression symbolized by Keq or Kc has the reactants and products written in brackets [ ] to represent molar concentration. The reaction N₂O₄ 2NO₂(g) according to the law of mass action would be written as:

However since both species of the reaction are gases, it is possible to write an equilibrium expression based upon the partial pressure of each gas

The two expression, Kp and K_c, are not numerically the same. To convert from one equilibrium constant to the other requires a correction factor where n =( moles of gaseous products - moles of gaseous reactants). To convert K_c to K_p use the equation

.

The equation is derived from the K_p for a general reaction, aA(g) bB(g) where a and b are stoichiometric coefficients. The equilibrium expression for the above reaction in terms of concentration units (mols/L) is written as

.

The same equation using partial pressures of both A and B is formulated as

Both of these are legitimate ways to express the equilibrium of the equation aA(g) <=> bB(g) since A and B are gases.

To derive the equation begin with the equilibrium expression for and substitute the values of P_A and P_B shown here as

into the equilibrium expression to obtain the following formula

Since moles/volume are concentration terms then the equation is reduced to

Where (b-a) is the change in moles of gaseous components written as Dn which renders the equation for converting K_p to K_c as

. Note that R can be expressed as 0.082 L atm/mol K, so the equation can be amended to be .