What is the concentration of the phosphoric acid solution?

Question

You prepared a solution of phosphoric acid for use in a series of precipitation experiments. To determine its concentration, you have titrated 26.00mL of the phosphoric acid solution to the second equivalence point with 43.95mL of 0.3185M KOH(aq). What is the concentration of the phosphoric acid solution?

Answer 1

Phosphoric acid = H₃PO₄

H₃PO₄ + KOH ==> KH₂PO₄ + H2O ... first equivalence point

KH₂PO₄ + KOH ==> K₂HPO₄ + H2O ... second equivalence point

Overall reaction to the second equivalence point is

H₃PO₄ + 2KOH ==> K2HPO4 + 2 H2O

moles KOH used: 43.95 ml x 1 L / 1000 ml x 0.3185 mol / L = 0.013998 mols KOH used

moles H₃PO₄ present: 0.013998 mols KOH x 1 mol H₃PO₄ / 2 mol KOH = 0.006999mols H₃PO₄ present

Concentration of H₃PO₄: 0.006999 mols / 26.00 ml x 1000 ml / L = 0.2692 mols H₃PO₄ / L = 0.2692 M