Chapter 4.7, Problem 5CYU

Vitamin C, ascorbic acid (C₆H_gO₆)(molar mass 176.1 g/mol), is a reducing agent. One way to determine the ascorbic acid content of a sample is to mix the acid with an excess of iodine,

C₆H_gO₆(aq) + I₂(aq) + H₂O(l) → C₆H_gO₆(aq) + 2 H₃O⁺(aq) + 2 I^–(aq)

and then titrate the iodine that did not react with the ascorbic acid with sodium thiosulfate. The balanced, net ionic equation for the reaction occurring in this titration is

I₂(aq) + 2 S₂O₃^2–(aq)→2 I^–(aq) + S₄O₆^2–(aq)

Suppose 50.00 mL of 0.0520 M I₂ was added to the sample containing ascorbic acid. After the ascorbic acid/I₂, reaction was complete, the I₂ not used in this reaction required 20.30 mL of 0.196 M Na₂S₂O₃ for titration to the equivalence point. Calculate the mass of ascorbic acid in the unknown sample.