#"Concentration"="Moles of solute"/"Volume of solution"#.
Common hydrogen halides are #HCl(g)# or #HI(g)#; these dissolve in water to give #"hydronium or acidium ions.........."#
#HCl(g) + H_2O(l) rarr H_3O^+ + Cl^(-)(g)#
The #H_3O^+# representation is more a conception than an actuality. As far as anyone knows it is cluster of water molecules with an extra #H^+# associated with the cluster.
The following is taken from an answer to a related question.
#HCl(g)# is a source of hydronium ion, #H_3O^+# in aqueous solution.........
We may take a tank of #HCl(g)#, and we can bleed it in to water to give an AQUEOUS solution that we could represent as #HCl(aq)# OR #H_3O^+# and #Cl^−#. In each case this is a REPRESENTATION of what occurs in solution.
As far as anyone knows, the actual acidium ion in solution is
#H_5O_2^+# or #H_7O_3^+#, i.e. a cluster of 3 or 4 water molecules with an EXTRA #H^+# tacked on.
Note that the #H^+# is quite mobile, and passes, tunnels if you like, the extra #H^+# from cluster to cluster. For this reason both #H^+# and #HO^-# have substantial mobility in aqueous solution.
#H^(+) + e^(-) rarr 1/2H_2(g)#
Depending at which level you are at (and I don't know!, which is part of the problem in answering questions on this site), you might not have to know the details at this level of sophistication. The level I have addressed here is probably 1st/2nd year undergrad.........
