Let's suppose we start with:

	initially
Red	10
Yellow	5
Green	1
Blue	0

Here Y is the limiting reagent. Thus the forward reaction will proceed until we run out of Y. That will happen when we have reacted 5 pairs of R and Y molecules, forming 5 pairs of G and B molecules.

So: we began with 10 R, 5 Y, 1 G and zero B. We react 5 R with 5 Y. That destroys 5 of the 10 R, and all 5 of our Y, leaving us with 5 R and zero Y. The reaction of 5 R with 5 Y produces 5 G and 5 B, and since we started with 1 G and zero B, we thus end up with 6 G and 5 B. Summarizing:

	initially	finally
Red	10	5
Yellow	5	0
Green	1	6
Blue	0	5

You can see there are two important considerations in figuring out the final concentrations. The first is how much of each reagent we begin with. The second is the proportion in which they are used up. This proportion is called the stoichiometry of the reaction. In this case the proportion (stoichiometry) is that we use 1 R for each 1 Y we use. But there are other possibilities in real life -- a pair of reacting molecules could form just one product molecule, or three or more. Or a single reactant molecule could produce two product molecules by splitting, and so on.

Try this reaction yourself in the applet. Set the initial concentrations to be exactly as they appear in the first table above, and then push the restart button. Wait until no more reaction takes place, then count the number of molecules of each type remaining.

Also try testing my assertion that the forward reaction rate constant doesn't change the final concentrations -- set k_f to different values and see whether the equilibrium number of R, Y, G and B molecules changes or not. Am I correct? If so, what does changing k_f do? Anything at all?