Question

A 25.00 mL sample of the triprotic acid phosphoric acid, H₃PO₄, requires 31.15 mL of 0.2420 M potassium hydroxide, KOH, for titration to reach the second equivalence point. What is the concentration (in molar) of phosphoric acid?

Answer 1

If it requires 31.15 mL of 0.2420M KOH for the titration to the second equivalence point.
Then it required 1/2 that to get to the first equiv point (15.575 ml of KOH)

find moles in that 15.575 ml of KOH
0.015575 litres * 0.242 mol/litre = 0.003769 moles of KOH

by the equation for that first equiv point
KOH + H₃PO₄ --> KH₂PO₄ + H₂O
0.003769 moles of KOH reacts with an equal number of moles of H₃PO₄ = 0.003769 mol H₃PO₄

find the molarity of the H₃PO₄(aq)
(0.003769 mol H₃PO₄) / (0.02500 litres) = 0.1508 Molar H₃PO₄