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 35 mL of 0.0107 M KIO3 solution was added, and the mixture titrated to a starch endpoint. The titration required 12 mL of 0.0790 M thiosulfate solution.

Answer 1

The no. of moles of KIO3 = (35/1000) L * 0.0107 mol/L = 0.0003745 mol

KIO₃ + 5KI + 6H⁺ ----> 3I₂ + 3H₂O + 6K⁺

Therefore, the no. of moles of I₂ = 3*0.0003745 = 0.0011235 mol

The no. of molesof Na₂S₂O₃ = (12/1000) L * 0.079 mol/L = 0.000948 mol

2Na₂S₂O₃ + I₂ ----> Na₂S₄O₆ + 2I^-

The no. of moles of I₂ consumed in the above reaction = 0.000948/2 = 0.000474 mol

Now, the no. of moles of I₂ remaining = 0.0011235 - 0.000474 = 0.0006495 mol

C₆H₈O₆ + I₂ ----> C₆H₆O₆ + 2I^- + 2H⁺

Therefore, the no. of moles of ascorbic acid in the vitamin C tablet = 0.000474 mol

i.e. The mass of ascorbic acid in the vitamin C tablet = 0.000474 mol * 176 g/mol = 0.083 g or 83 mg

Hence, the weight percent of ascorbic acid in a tablet of vitamin C = 83%