HCl is a strong acid which means that it dissociates completely into ions when in solution. These ions are the hydronium and the chloride ions. The dissociation reaction would be:
HCl = H+ + Cl-
[H+] = 0.250 mol / L (0.030) ( 1 mol H+ / 1 mol HCl) = 0.0075 mol H+ / L
pH = -log [0.0075] = 2.12
