The equilibrium constant has a unique value and remains constant at constant temperature. To see how we calculate the equilibrium constant let's see an example

Example 1

The reaction above is allowed to reach equilibrium, at which point the concentration of each species was measured. Calculate the equilibrium constant given the following concentrations at equilibrium.

Step 1 Write down the equilibrium expression.

Step 2 Calculate the concentration of each species. In this case the concentrations are given.

Step 3 Calculate the equilibrium constant

Note the units of the equilibrium constant. The units will depend on the equilibrium expression which is derived from the original equation.