Hello, Itzel,
pH is defined as the negative log of the hydrogen ion (H+) concentration. HCl dissolves completely so any solution of 0.41M HCl will all dissociate into H+ and Cl- ions. Let's assume a random volume. We can pick anything because the concentration is expressed as moles/liter.
I like a liter, so let's use that. This mean we had 0.41 moles of HCL that came apart to make 0.41 moles of both H+ and Cl-.
The H+ concentration is thus 0.41 M. The negative log of this number is 0.39, so pH = 0.39.
I hope this helps,
Bob
