`K_c = [P]^y/[R]^x`

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The Equilibrium Constant formula, K_c = [P]^y / [R]^x, is used to calculate the reaction quotient when a reaction has reached equilibrium. Calculating equilibrium constants can help determine the composition of a system while it is at equilibrium. A typical equation, xR = yP, can be used to represent the equilibrium constant formula because [products] are always in the numerator, with [reactants] in the denominator. Therefore, the product "P" belongs in the numerator with reactant "R" in the denominator. Each variable is raised to its respective equation coefficient, y and x. The input variables are coefficients x and y (unit-less) and the concentrations of [P] and [R] in mols/L.   

Setting K against Q can provide useful information about which direction a reaction will proceed. It gives five possibilities:

• Q = K      The system is at equilibrium and does not shift left or right
• Q = 0       The system will shift forward/to the right (towards products)
• Q < K       There are more reactants than products, thus the system will shift to the right (towards products)
• Q > K      There are more products than reactants, thus the system will shift to the left (towards reactants)
• Q = infinity (Q >>> K) The system will shift left (towards reactants)

Example:

aA + bB = dD + eE

K_c = [products] / [reactants]

K_c = [E]^e [D]^d / [B]^b [A]^a

References

Source: Wikipedia