# How to calculate keq from pka вЂ ‹вЂ‹

### Calculation of the GG concentrations and the equilibrium constant Kc

To explain the calculation of the equilibrium concentration, we take the reaction as an example:

The following formula applies to the calculation of the equilibrium constant:

If Kc >> 1, the products are in equilibrium.

If Kc << 1, the equilibrium is with the educts.

In the following example only the initial concentrations of the starting materials and the equilibrium concentration of the product are known. In order to be able to calculate the equilibrium constant, however, one needs the equilibrium concentrations of all substances involved. To do this, first enter the given values ​​in a table.

 material Initial concentration 0 modification Equilibrium concentration

Since we have an initial concentration of HI with 0 mol / l, we can also specify the equilibrium concentration directly with the change.

We divide the change in the concentration of HI by 2 to get the change in the other two substances. Because there are only half a molecule of H per molecule of HI2 and I.2 needed. The change is then subtracted from the initial concentration in order to obtain the equilibrium concentration. The two values ​​are added to the table.

 material Initial concentration 0 modification Equilibrium concentration

Now that we have all the required values, we can use the formula to calculate the equilibrium concentration.

Created by Noah S. with minor changes by Mr. Ecker 23.5.2013.