M, or molarity is the term used to tell you how many Moles of atoms or molecules is in a solution. A mole is equal to 6.023 X 1023 atoms or molecules. So a 1M solution of anything has 6.023 X 1023 atoms or molecules of it in it, in this case, HCl. A 3M solution has 3 times this much.
[There is a reverse formula to find the maximum Molarity of say 100% HCL but I can't think of it just yet. Does this help.]
[There is a reverse formula to find the maximum Molarity of say 100% HCL but I can't think of it just yet. Does this help.]