Strong acids completely dissociate in aqueous solution.HCl(aq) → H+(aq) + Cl-(aq)This means that the acid concentration and the hydrogen ion concentration are the same. [H+] = [HCl] so the pH of 0.2moldm-3 of HCl is -log100.2 which is 0.699 to 3 s.f.Sulfuric acid is a dibasic acid.H2SO4(aq) → 2 H+(aq) + SO42-(aq)This means that the hydrogen ion concentration [H+] is twice the concentration of the acid. This is because two hydrogen ions are released for each molecule of acid. So the pH of 0.2moldm-3 of H2SO4 is -log10(0.2x2)which is 0.398 to 3 s.f.