The Equilibrium Law 
For any general reaction system the value of K_{eq}
is called
the equilibrium constant for the reaction. for example:

3 H_{2}(g) + N_{2}(g) <=====> 2 NH_{3}(g)
+ heat

K_{eq} = [NH_{3}]^{2}
[H_{2}]^{3}[N_{2}] 
The above equation is called the Equilibrium Law
Expression and
it is a general form of the Law of Mass Action.

Law of Mass Action 
In a system at equilibrium, at a fixed temperature, the
product
of the equilibrium concentration of the products divided by the product
of the concentrations of the reactants, each being raised to the
coefficient
of the substance in the equation, must be equal to a constant.

This law can only be applied to ideal gases and solution.
(There
are some reactions that do go all the way).

Here is another example of an equilibrium law expression.

H_{2}(g) + I_{2}(g)
<=====> 2 HI(g)
+ heat


Write equilibrium law expressions for each of the
following reactions:

a) 2 NO(g) + O_{2}(g) <=====> 2
NO_{2}(g)
b) 2 SO_{2}(g) + O_{2}(g) <======> 2 SO_{3}(g) c) SO_{2}(g) + ½O_{2}(g) <======> SO_{3}(g) d) SO_{3}(g) <======> SO_{2}(g) + ½O_{2}(g) e) 4 HCl(g) + O_{2}(g) <======> 2
H_{2}O(g)
+ 2 Cl_{2}(g)

Some of the equations above are almost carbon copies of each other except for the values of the coefficients. A couple of general rules should help. 
K_{eq(forward)} 
Any reaction written normally can have a K_{eq }value.
But
does this K_{eq }value stay the same if the reaction is
flipped
over and written in reverse. No, if the equation flips the K_{eq }value
becomes the inverse.
At any given temperature there will be numerous [ ] values that will satisfy a constant K_{eq} value. 