[amk0713]

Hi, there.

I am having a little bit of trouble understanding the purpose of the equilibrium constant. What does it tell us? I understand that it's a ratio, but why not just use stoichiometry to figure out the ratio of of reactants to products? Just a side note: I am extremely mathematically deficient.

[UG]

The magnitude of K tells us the position of the equilibirum and provides a measure of the extent of the reaction in either a forward or backward direction.

[Loyal]

The information that can be garnered from the equilibrium constant is enormous.   The simple information tells you if a reaction is product or reactant favored.  You can also get energies associated with the reaction, pH in complicated systems, quantities of all species in the reaction, and many other useful pieces of information.    It also allows you to predict how a reaction will act when pressure and temperature change. 

Also when you start to see more complex problems such as competing reactions, reactions that occur in a series, or other cases the constant allows you to calculate all of the possible species in solution.    Some of those techniques may be a little more advanced than your current chemistry class (I am not aware of what level you are taking), but they do have their purposes.    

[Borek]

why not just use stoichiometry to figure out the ratio of of reactants to products?

Because reactions not always go to completion.