# Derivation of the Equilibrium Constant

I'm trying to understand this: http://en.wikipedia.org/wiki/Equilibrium_constant#Derivation_from_Gibbs_Free_Energy
And there are few steps that I don't get. My first question is probably a stupid one, but where does the $$\Sigma$$v$$_{j}$$A$$_{j}$$=0 come from? What exactly is it summing up, the number of atoms? Are you assuming that the products side is negative? (Because I don't see how else you would get zero) And why does it only go up to n?

I'll ask my other questions after I get some replies because it's possible I'll figure them out after I read the replies.

Borek
Mentor
where does the $$\Sigma$$v$$_{j}$$A$$_{j}$$=0 come from? What exactly is it summing up, the number of atoms? Are you assuming that the products side is negative? (Because I don't see how else you would get zero) And why does it only go up to n?
My guess is it is just a mass conservation, and yes, products are negative.

My guess is it is just a mass conservation, and yes, products are negative.
Ok, but why does it only sum up to n? The first equation in the derivation shows the first product being n but then shows more products after that. If it was mass conservation, wouldn't it have to sum passed n?

Borek
Mentor
Where does it sum only to n? It sums for all j.