6. pH of a Solution of a Strong Acid

When a strong acid like HCl is dissolved in water, nearly all HCl-molecules will donate a proton:

HCl(aq) + H₂O(l) H₃O⁺(aq) + Cl^-(aq)
or
HCl(aq) H⁺(aq) + Cl^-(aq)

So [H₃O⁺] or [H⁺] will be equal to the original concentration of the acid HCl:

For instance: 0.1 mol/L HCl(aq) [H⁺] = 0.1 mol/L pH = -log 0.1 = 1.

pH + pOH = 14 pOH = 13.

In this case the amount of hydrogen ions contributed by water is negligible compared with the 0.1 mol/L from the ionization of HCl. In pure water this amount is only 10^-7 mol/L. Due to the presence of an abundance of hydrogen ions delivered by the strong acid HCl, the water equilibrium will shift completely to the left.

Summary

pH of a solution of a strong acid HA in water

[H+] = coHA Calculate pH