What is an equilibrium constant?
At equilibrium, all the concentration of reactant and products are constant. We use the concentration to calculate the equilibrium constant. 

equilibrium constant (K_eq) - The product of the molar concentrations of the products for a chemical reaction, each raised to the power of its respective coefficient in the equation, divided by the product of the molar concentrations of the reactants, each raised to the power of its respective coefficient in the equation.

This might seems pretty confusing right now, let's look at some examples and see how we can use this equilibrium constant.

The equation for equilibrium constant is:

where,

Notice a few rules,
1. [A], [B], [C], and [D] = molar concentration at equilibrium.
2. products are always in the numerator, and reactants are always in the denominator.
3. coefficients in the chemical equations are the exponents.
4. Solids and liquids are ignored.
5. No units for K_eq
6. Concentrations are always in moles per liter.

Example: What is the equilibrium constant expression for ?
In this case, notice that H₂O is in the gaseous form. If it was in the liquid or the solid state, then we won't use it in the K_eq.
After putting everything into the equation, we get the answer:

Example: What is the equilibrium constant expression for ?
This is slightly different than the above example. In this case, there is a solid in the equation. Looking back at rule #4; we will need to omit the solid from the equilibrium expression.

• Note that the value of the equilibrium constant depends on the temperature. If the temperature change, then the concentration will change also. Thus we will have a different valve. (The equation for the equilibrium constant is still the same).
  

How to calculate the value of an equilibrium constant?

Once we have the equilibrium constant expression from the chemical equation. We can just plug in the molar concentrations at equilibrium to get the value of the equilibrium constant.

Example: At the temperature 400^° , the equilibrium molar concentrations are:
[CO] = 0.613
[H₂] = 1.839
[CH₄] = 0.387
[H₂O] = 0.387

for the reaction 
What is the value of the equilibrium constant?

Step 1: We have to first figure out the equilibrium constant expression.
Which is:

Step 2: Plug in the numbers and solve:

= 0.0393 (notice there is no unit)

To sum it all up, the equilibrium constant gaves the relative amounts of products and reactants at equilibrium. We can use this number to figure out all sorts of questions related to equilibrium.