K_{c} Calculations

By using the concentrations of reactants and products at equilibrium, K_{c} can be determined.

For example:

1.0 moles of each A and B_{2} are allowed to reach equilibrium at temperature, T. 0.5 moles of C are present at equilibrium, determine K_{c}.

Write out the K_{c} expression.

If we know the starting amounts of A and B_{2}, and the final amount of C, we can find the amounts at equilibrium.

Two moles of C are produced for every one mole of each A and B_{2} that react. Therefore, if 0.5 moles of C are present at equilibrium, 0.25 moles of each A and B_{2} must have reacted.

One mole of each A and B_{2} are present at the start. If 0.25 moles of each react together, 0.75 moles of each are left over. This means that at equilibrium we have 0.75 moles of each A and B_{2}, and 0.5 moles of C.

These values can now be used in the K_{c} expression to find the value of K_{c}.

Here, the number of moles of A, B_{2} and C are used instead of concentrations. This is because the volume of the system does not change, so the volumes cancel out.

There are no units for K_{c} in this example because the units for concentration also cancel out.