Question

1. What is the initial pH expected for a 0.1 M solution of phosphoric acid (H3PO4)? For 30.0 mL of 0.1 M H3PO4 (aq), what volume of 0.1 M NaOH (aq) is required to fully titrate all three protons to their end points?
2. In the titration of a weak acid with a strong base, how is the half equivalence point determined and what is its significance? How are the pKa and Ka of the weak acid determined from the half equivalence point?

Answer 1

pH of 0.1 M H3PO4

Ka1 = 0.00708, H+ = √(0.00708 × 0.1) = 0.0266M

pH = -log [H+] = 1.575

Volume of 0.1 M NaOH will 3 times then the volume 0.1M of H3PO4, so the volume of H3PO4 is 90 ml.

In the titration of a weak acid with a strong base, The half-equivalence point of an acid-base titration is the point at which the concentration of an added base is equal to half of the original concentration of the acid. The half-equivalence point is also known as the midpoint of a titration. Because of incomplete dissociation of acid, the reaction is in equilibrium, with the acid dissociation constant Ka which is specific to that acid point. Therefore at the half-equivalence point, pH is equal to the pKa,