CHEM1312H - pH of Pure Acids

strong acids

-if the ionization equilibrium lies far to the right, the acid is strong (can lose protons easily or product-favored) (K>>1)

-HCl and HNO₃ are monoprotic (contains one ionizable proton)

-H₂SO₄ is diprotic (contains two ionizable protons)

calculating the pH of strong acid solutions

-since strong acids are considered strong electrolytes in solution, the [H₃O⁺] in a strong acid solution is equal to the concentration of the strong acid

-pH of a strong acid can be calculated directly from the concentration of the strong acid (this includes concentrated solutions of H₂SO₄)

calculating the pH of strong acid solutions exceptions

-the only exceptions are extremely dilute (< 10^-6) strong acid solutions

-autoionization in pure water produces a H₃O⁺ ion concentration of 1.0 × 10^-7M and will have a contribution to the hydronium ion concentration

-the pH of extremely dilute solutions is slightly lower than 7

-calculations are too complex)

weak acids

-if the equilibrium lies far to the left (partially ionizes) the acid is weal (does not lose protons easily) (K<<1)

-can be monoprotic, diprotic, or triprotic

-carboxylic acids such as acetic acid are also weak acids (contains a carboxyl group)

determining the pH of a mixture of a strong acid and weak acid

-three sources contributing to the H₃O⁺ concentration: the strong acid, weak acid, and autoionization of water

-in this case the contribution of the weak acid is negligible, and we consider its contribution equal to the autoionization of water (also negligible)

-according to le chateliers principle, the formation of hydronium ion by the strong acid causes the weak acid to ionize even less than it would in the absence of the strong acid

calculating the pH of weak acid

-the hydronium ion concentration is not equal to the concentration of the weak acid as it partially ionizes, and pH of a weak acid is calculated using [H₃O⁺] at equilibrium

-steps: write balanced equation, make ice table, substitute the equilibrium concentrations into the equilibrium equation for the principle reaction and solve for x, calculate the concentrations of all reactants and products, calculate pH