How to calculate K?

When a question asks you to calculate the Keq of a reaction

For example, 5.0 M ammonia pumped into a container containing water and left to react.  After a few days, measurements are taken to see how much ammonia and water and remaining and how much product is produced.  At equilibrium, 3.6 M of ammonia was still present and 3.0M of ammonium and 3.0M of hydroxide was produced.

Step 1 - Write the Keq equation

Remember to always write the Keq equation with the products in the numerator and the reactants in the denominator.

[NH4+(aq)][OH-(aq)]
Keq =
[NH3(aq)]

Step 2 - Sub in known values

Now that the Keq equation is written, all the known values need to be entered in.  The only thing to watch out for when you sub in the value is to make sure that all the values have the correct units, mol/L.  Sometimes questions have the values still in grams per litres, and in this case the grams needs to be divided by the molar mass of the molecule to turn grams into moles.

[3.0 M][3.0 M]
Keq =
[3.6 M]

Remember, another way to write "mol/L" is with a capital "M".

Step 3 - Solve for the Keq

[3.0 M][3.0 M]
2.5 =
[3.6 M]
Having a Keq value of 2.5 means that the products were favored, even if it was only a little larger than a Keq of 1.

Video Examples

The video below contains two examples of solving Keq problems.

Jūs užbaigėte 83% pamokos
83%