Actually, it's not; and I can prove it by using the example as a whole.No, it won't. Marius is right. The overall result depends on the totality of the unpredictable factors. Your equation is merely an approximation.
Let's take one liter of water, and add 3.00 mol NaCl at point X. At time Y+ (any time after the net movement has exceeded the distances availiable) I can take the whole sample, and demonstratably show that we now have 3.00M NaCl(Aq). It cannotbe more or less, due to the conservation principle.
What you're forgetting is that molarity is merely a statement of concentration, and not position. As I stated, the math to describe the exact positions of each Na+ ion does not exist today. However, we can predict the overall concentration *without* having to calculate the exact positions. The exact positions of the particles don't really matter.
Let's use blood as an example. Marius will gleefully pounce on any errors I make, so we'll use him for a fact checker. Okay, so when O2 enters the bloodstream at the pulmonary capillaries, the diffusion gradient causes O2 to be exchanged with CO2, saturating our blood with oxygen. This is pretty basic stuff, so far.
However, that amount of O2 isn't sufficient for our needs. What happens is that the O2 binds to Hemoglobin, which effectively takes it out of the gradient. So, more O2 diffuses in, gets bonded to hemoglobin, etc, etc. Given standard conditions, I can then predict how much O2 is being absorbed by the capillaries at any given moment. (The Starling equasion, basically.)
Now, it doesn't matter where in the alveoli the O2 happens to be during this exchange. If you stop and think about it, though, it should be highest at the points furthest away from the epithelium, where gas exchange does not occur. But even with a mismatch like that, I can still correctly predict the rate of exchange.
See what I'm getting at? I don't need to know the exact number and position of molecules to provide some very exacting results. In fact, it will be exact; you should be aware of the principle of significant figures.