Question

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 20. mL of 0.0138 M KIO₃ solution was added, and the mixture titrated to a starch endpoint. The titration required 9.0 mL of 0.0750 M thiosulfate solution.
Weight =

Answer 1

The balanced equations, involved here, can be written as follows.

IO₃^- + 8I^- + 6H⁺ 3I₃^- + 3H₂O ........ Equation 1

I₃^- + 2S₂O₃^2- 3I^- + S₄O₆^2- ....... Equation 2

Ascorbic acid (C₆H₈O₆) + I₃^- + H₂O C₆H₆O₆^2- + 3I- + 2H⁺​ ....... Equation 13

The no. of millimoles of IO₃^- = 0.0138 mmol/mL * 20 mL = 0.276 mmol

The no. of millimoles of I₃^- = 3*no. of millimoles of IO₃^- = 3*0.276 mmol = 0.828 mmol

The no. of millimoles of thiosulfate (S₂O₃^2-) = 0.075 mmol/mL * 9 mL = 0.675 mmol

The no. of millimoles of I₃^- remaining to react with ascorbic acid = 0.828 - (0.675/2) = 0.4905 mmol

From Equation 3, the no. of millimoles of ascrobic acid should be the same as 0.4905 mmol

The no. of millimoles of vitaminc C = 100 mg/(176 g/mol) = 0.5682 mmol

Therefore, the weight percent of ascorbic acid in the tablet of Vitamin C = (0.4905 mmol/0.5682 mmol)*100 = 86.3%