If a weak acid HA with Ka = 1×10^-5 is titrated with a strong base, is the pH at the equivalence point less than, equal to, or greater than 7?

If a weak acid HA with Ka = 1×10^-5 is titrated with a strong base, is the pH at the equivalence point less than, equal to, or greater than 7?

• A. Less than 7
• B. Equal to 7
• C. Greater than 7
• D. Cannot be determined

Answer: (C)

At the equivalence point, all of the weak acid has been neutralized, leaving its conjugate base A− in solution. This A− undergoes hydrolysis: A− + H2O ⇌ HA + OH−. The extent of this hydrolysis is governed by Kb, where Kb = Kw/Ka. With Ka = 1×10−5, Kb = 1×10−9, which is enough to generate OH− and push the pH above 7. The exact pH depends on the concentration, but the solution will be basic (greater than 7) at the equivalence point.

At the equivalence point, all of the weak acid has been neutralized, leaving its conjugate base A− in solution. This A− undergoes hydrolysis: A− + H2O ⇌ HA + OH−. The extent of this hydrolysis is governed by Kb, where Kb = Kw/Ka. With Ka = 1×10−5, Kb = 1×10−9, which is enough to generate OH− and push the pH above 7. The exact pH depends on the concentration, but the solution will be basic (greater than 7) at the equivalence point.