[nikoskaranikolasp]

I struggle to understand the logic behind problems like this:
In 4L of NaOH 0,2M solution we add 1L of HCL 0,3M solution. Find the pH of the final solution.
The answer is we add HCL and NaOH and find the pH of the final solution from the substances left. Here's my problem. A solution of HCL 0,3M has no HCL left since it has all become Chloride Ion and Hydronium, right? The same goes for NaOH. So where do they find the NaOH and HCL to add them? Thanks in advance.

[mjc123]

You are right, a "solution of NaOH" contains Na⁺ and OH^- ions, not NaOH molecules, and a "solution of HCl" contains H⁺ and Cl^- ions, not HCl molecules. But it's simpler to say and write "a solution of NaOH" than "a solution of Na⁺ and OH^- ions". So we do. After all, it's what you get when you dissolve NaOH in water, so it's justifiable to call it a NaOH solution, as long as you are aware of what that is chemically.
The reaction that takes place is not between NaOH and HCl molecules, but between H⁺ and OH^- ions. Na⁺ and Cl^- are called "spectator ions"; they are present but don't take part in the reaction.

[AWK]

Note, HCl neutralize a part of NaOH (or hydronium ions react with hydroxide ions). Moreover, you dilute your solution. Taking these facts into account you can calculate an approximate pH of a solution after neutralization (and dilution).
The more exact calculation should take into account also the ionic strength - though this correction. in this case, may change pH by 0.1 to 0.2 pH unit only.