Here we can say that when all equilibrium concentrations of reaction are known then our equilibrium constant K and be calculated. In addition to this we can also say that your equilibrium constant can also be used to calculate 1 missing equilibrium concentration.

If we take a look here at this example question, it says what is the value of the equilibrium constant KC for the reaction below if the equilibrium mixture contains 0.255 molar of methane, 1.10 molar of carbon dioxide, 0.388 more of carbon monoxide and 0.250 molar of hydrogen gas. Now here they're telling us this is an equilibrium mixture. So these are all equilibrium concentrations.

We say that K equals products over reactants. Remember, we exclude solids and liquids from this expression. Since everything is a gas, everything is going to be included. We're going to have here products overreacted. So that's CO2. So if it's coefficient is a 2, that's going to be squared times H2. Coefficient is 2, so that's also squared divided by methane, which has a coefficient of 1, so we don't need to include it times the concentration of CO2.

Now we're going to plug in the values given to us for each of the compounds. We have carbon monoxide as being 0.388 molar which is going to be squared times 0.250 molar for H2 which is also squared divided by. Here we have 0.255 molar for methane and we have 1.10 molar for carbon dioxide. We plug this into our calculators and what we will get initially, and I'm going to give a long string of numbers and then we're going to round it down.

So we're going to say here we have 0.033543672. Our equilibrium constant K has no unit, so we're not going to plug in molarity or anything. It's just this value Here. We're going to say within our question, all the values given to us have three significant figures. So we're going to give K 3 significant figures. This means that our final answer is going to be 0.0335. This represents the answer for our equilibrium constant K from the given example question.