A solution containing 1.90 g of a diprotic acid H₂C₃H₂O₄ (malonic acid) was titrated with 45.5 mL of NaOH to reach the second equivalence point. What is the concentration of the NaOH solution?

Question

(MW_malonic acid = 104.06 g/mol)

Answer 1

We can represent the diprotic acid as H₂A (2 ionizable hydrogens)

H₂A + 2NaOH ==> 2H₂O + Na₂A .. balanced equation for titration with NaOH

moles H₂A present = 1.90 g x 1 mol / 104.06 g = 0.01826 moles

moles NaOH to reach 2nd equivalence point = 0.01826 mol H₂A x 2 mol NaOH / mol H₂A = 0.03652 mol

Concentration of NaOH = 0.03652 mols / 45.5 mls x 1000 ml / L = 0.803 mol / L = 0.803 M