Question

A 32.00 mL sample of an unknown H3PO4 solution is titrated with a 0.110 M NaOH solution. The equivalence point is reached when 28.23 mL of NaOH solution is added.

What is the concentration of the unknown H3PO4 solution?

Answer 1

You first of all need to write a balanced chemical equation for the reaction of sodium hydroxide and phosphoric acid. You know that you have 0.110 M and 0.02823 L of sodium hydroxide. You multiply 0.110 M times 0.02823 L to get 0.0311 mol of sodium hydroxide and then you multiply by the mol ratio of 1 mol of phosphoric acid to 3 mol of sodium hydroxide to get the mol of phosphoric acid. You then divide 0.00104 mol of phosphoric acid by 0.03200 L to get 0.0325 M.