Question

250.00 mL sample

0.1M NaOH used to titrate h3po4 and h2po4 sol.

first equivalence point: reached at 10.5mL at pH 4.95

Second equivalence point: 37.5mL at pH 9.83

Titration stops at 41.5 mL at pH 11.01

H₃PO₄  + H₂O ↔ H₂PO₄^- + H₃O⁺   pKa₁ = 2.1

H₂PO₄^- + H₂O ↔ HPO₄^2- + H₃O⁺ pKa₂ = 7.1

  HPO₄^2- + H₂O ↔ PO₄^3-  + H₃O⁺   pKa₃ =12.3

Find: concentration of NaOH in sol at the equivalence point

NaOH [M] at Veq1:

NaOH [M] at Veq2:

Answer 1