Molar concentration...

Guys, we're not here to bash each others intelligence, nor to go off topic and post posts that doesn't contribute to the thread at all. Get this train on the right track before I have to stop it.

Sorry if it seemed that way, but I pretty much read the first post, glanced through the thread, replied with my solution, and then reread through the whole thread to realize that the problem was already solved by other people. That's the only reason my post seemed empty. You can check the edit logs and see what I had there originally.

After going through the problem again, I believe Moon's equation tells me the concentration of HCl moles after adding more water but not the concentration of the whole solution.

I don't understand why you would take the concentration of a pure substance, but that's pretty much just the density. It's pretty much saying that instead of dissolving the pure substance in some other existent particles, you're "dissolving" the pure substance in space. That's why dividing the density of pure liquid water by its molar mass returns the molar concentration of pure liquid water in space. You already did that. To generalize this concept beyond water, a formula that computes the total molar concentration is:

c_i = ∑(n_i) / ∑V

where
(n_i) = d/M.
d = density
M = molar mass

In this case, since HCl doesn't affect the volume of the solution significantly, the total molar concentration is just:

c = (n_HCl + n_H2O) / (V1 + V2)

Still not sure why you would ever use this. *Shrug*