Calculating Keq

Keq expresses the relative concentrations of reactants and products in an equilibrium. This is the equilibrium constant. Keq can be calculated as follows :

 

The [ ] represent the concentration in moles/L

If the molecules in the reaction have coefficients, these will become exponents for the molecules in the Keq formula. For example, if 2A + B <----> C + D the formula would look like this : Keq = [C][D] / [A]²[B]. Remember, this formula always calls for products OVER reactants.

Now, lets look at an example:                                

 2NO_{2 <-->} N₂O4

[N₂O4] at equilibrium = 0.00140 M

[NO_{2] at equilibrium = 0.0172 M}

_Keq=[N₂O4] / [NO₂]²  

Keq=(0.00140)/(0.0172)²

Keq=4.73