Analytical Chemistry: Calculate Weight Percent

Question

For the problem below I keep getting 102 but the online homework is saying it is wrong. It says you need to go to 4 significant figures so I may be calculating it wrong. Thanks for the help.

Calculate the weight percent of ascorbic acid in a tablet of Vitamin C from the following data:

A 100 mg sample of a crushed Vitamin C tablet was dissolved in 40 mL of H₂SO₄ and 20 mL of water. Two grams of KI and 25 mL of 0.0135 M KIO₃ solution was added, and the mixture titrated to a starch endpoint. The titration required 17 mL of 0.0510 M thiosulfate solution.

Weight = ___

Answer 1

The question is asking for the weight percent of ascorbic acid in the original sample.

I used the following reactions:

C₆H₈O₆ + I₂ ==> C₆H₆O₆ + 2I^- + 2H⁺ ... reaction (1)

IO₃^- + 5I^- + 6H⁺ ==> 3I₂ + 3H₂O ... reaction (2) (Na⁺ and SO₄^2- are spectator ions, and aren't shown)

I₂ + 2S₂O₃^2- ==> 2I^- + S₄O₆^2- ... reaction (3)

I ran through the calculations and arrive at an answer similar to yours, where I find 101.97 %, but how can the % be greater than 100%? It can't. Using 4 sig. figs., this would be reported as 102.0% which is what you obtained. I'm wondering if there is an error in the problem as written. And why they ask for 4 sig. figs. is beyond me since the 100 mg sample has only ONE sig. fig. If requested, I can submit my actual calculations, but not sure what that will accomplish. Maybe someone else will weigh in with a different approach.