What is the pH when 10.0 ml of 0.10 M NaOH has been added to 25.0 ml 0.10 M phosphoric acid?

Answer 1

You have 1 mmol of base and 2.5 mmol of a weak acid. In the end, you'll have 1.5 mmol of acid left and 1 mmol of H2PO4-, so you'll end up with a buffer situation. Use the Henderson-Hasselbach equation to determine the pH (volume will be irrelevant since both acid and base will be in the same solution). You'll use pKa1 for phosphoric acid, which is 7.5 * 10^-3 (pKa = 2.12)

pH = pKa + log (mmol H2PO4-/mmol H3PO4)
pH = 2.12 + log (1/1.5) = 2.12 - 0.18 = 1.94

There will be a negligible amount of H2PO4- that will dissociate to H+ and HPO4(2-), but that will not affect the pH noticeably.