Equilibrium constant

Equilibrium constant is a mathematical quantity which expresses the relationship between products and reactants of a reaction at equilibrium with respect to a specific unit. In other words, equilibrium constant is the reaction quotient of a chemical reaction at chemical equilibrium. The equilibrium constant can help us to understand whether the reaction tends to have a higher concentration of products or reactants at equilibrium. We can also use to Equilibrium constant determine if the reaction is already at equilibrium.
There are several different types of equilibrium constants that provide relationships between the products and the reactants of a chemical reaction at chemical equilibrium in terms of different units. Dissociation constants can be used as an example.
Equilibrium constant of a reaction.
For a general chemical equilibrium
the equilibrium constant can be defined by
where {A} is the activity of the chemical species A, etc. (activity is a dimensionless quantity). It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator.
For equilibria in solution, activity is the product of concentration and activity coefficient. Most chemists determine equilibrium constants in a solution with a high ionic strength. In high strength solutions, the quotient of activity coefficients changes very little. So, the equilibrium constant is defined as a concentration quotient:
Or, 
Here, formula_5 is the partial pressure of formula_6.
If, 
Then,
Here, formula_9 is the equilibrium constant in terms of partial pressure.
In this same process, 
For the reaction above,
So, the relation between formula_12 and formula_9 is,
Here, formula_15 the number of moles of gas on the product side minus the number of moles of gas on the reactant side in the balanced reaction