Le Chatelier's Principle?

Easy question. 12M HCl contains water thus increasing solution volume, BUT you are also adding 12M Cl-.... so if we have Na+Cl- in solution we can only have an equal amount of concentration of Na+ to concentration of Cl-. If we add the concentration of Cl- from the HCl, then we disrupt the equilibrium in favor of Cl-. To compensate for this addition, the crystallization of NaCl occurs.