Unit 8: Topic 7 - pH and pKa Flashcards
A weak acid HA and its conjugate base A- are both present in aqueous solution. If pH < pKa, what does this tell us about the concentrations of [A-] and [HA]?
The dissociation of the acid follows the reaction:
HA(aq) + H2O(l) –> H3O+(aq) + A-(aq), Ka.
At equilibrium, Ka = [A-][H3O+]/[HA]. Since Ka is small for a weak acid, the concentrations of [HA] and [A-] change very slightly from their original concentrations, so [H3O+] ≈ Ka * [HA]/[A-] is a very good approximation. Taking the negative log gives pH = pKa + log([A-]/[HA]), which is actually the Henderson-Hasselbalch Equation.
If pH < pKa, then using this equation, [HA] > [A-]. Similarly, if pH > pKa, then [A-] > [HA].
Explain why phenolphthalein can be used as an indicator for acid-base titrations.
Phenolphthalein is an acid. This compound is colorless when protonated, such as when it is present in acidic solution. However, in basic solution, the proton dissociates, and the deprotonated compound has a different color. This is useful in weak acid-strong base titrations, where the equivalence point has a pH of around 8.3, exactly the pH at which phenolphthalein changes color.
