Question

Q1: The phosphoric acid in a 100.00-mL sample of cola drink was titrated with 0.1025 N NaOH. If the first equivalence point occurred after 13.11 mL of base was added, and the second equivalence point occurred after 28.55 mL of base, calculate:

a.     the concentrations or molarity of H₃PO₄           {H₃PO₄ + OH^- à H₂PO₄^- + H₂O}

b.     The concentration of H₂PO₄^- in the cola sample. {H₂PO₄^- + OH^- à HPO₄^-2 + H₂O}

c.     How would CO₂ interfere with the titration of H₃PO₄?   {CO_2(g)   + H₂O à H₂CO₃}

Answer 1

Phosphoric acid in cola drink titrated with NaOH

a. moles of NaOH used to reach first equivalence point = 0.1025 M x 13.11 ml = 1.344 mmol

moles of H3PO4 = 1.344 mmol

concentration of H3PO4 = 1.344 mol/100 ml = 0.01344 M

b. moles of NaOH used to reach second equivalence point = 0.1025 M x 28.55 ml = 2.9264 mmol

moles of H2PO4- = 2.9264 mmol

concentration of H2PO4- = 2.9264 mmol/100 ml = 0.02926 M

c. CO2 makes the solution basic and it would enhance the dissociation reaction. So we would end up having more H2PO4- in solution.