If it is significantly different the exact doesn't matter much, as activity coefficients are far from unity.
Technically you should take into account solution density, calculate the solution mass, subtract masses of all ions present, whatever is left is water - and knowing volume you can calculate concentration.
So would the correct process to go about be: find the mass of the solution (number of moles of everything * Mr), find the density (mass/volume), and then ... the problem is that I don't know how many moles of H2O there are yet, and I can't work out the solution mass without that information.
e.g. let's say I add 20M HCl to water. (Assume HCl is strong, water is pure, the form of the H
+ ion in water is indeed H
3O
+, Kw = 1.00*10
-14). And yes, I know the pH of this is impossible to work out by theoretical means. But since H
3O
+ can be worked out easily, [H
2O] should also be possible to find?) how can I find the [H
2O] after addition of HCl then?