Question

A student was given an old tablet of vitamin C weighed 1.022 g to analyze. The tablet was dissolved in 700.0 mL water, and 35.0 mL aliquot of this solution required 45.6 mL of unknown molarity I₂ (stock A) solution to oxidize it. In another experiment, the student titrated 50. mL of that same I₂ (stock A) with 17.6 mL of 0.0100 M ascorbic acid. Calculate the mg vitamin C in the tablet.

Answer 1

ascorbic acid and I2 react in 1:1 ratio

Ascorbic acid moles reacted = M x V ( in L)

                               = ( 0.01 ) x ( 17.6/1000) = 0.000176

I2 moles reacted = 0.000176 , volume of I2 used = 50 ml = 0.05 L

Molarity of I2 = moles of I2 / volume of I2

             = (0.000176 / 0.05) = 0.00352 M

I2 solution used to titrate with vitamic C = M x V where V is 45.6 ml i.e 0.0456 L

           Moles of I2 used to titrate with vitamic C = 0.00352 x 0.0456 = 0.0001605

Moles of vitamin C = 0.0001605

Aliquot volume used = 35 ml =0.035 L

Molarity of vitamin C in aliquot = moles of vitaminc C / aliquot vol

                      = 0.0001605 /0.035 = 0.004586

Moles of vitamin C in 700 ml   ( which is where tablet completely dissolved) = M x V

           = 0.004586 x ( 700/1000) =0.00321

mass of vitamin C in tablet = moles x molar mass of vitamin C

           = 0.00321 mol x 176.12 g/mol

          = 0.565 g

Thus mass of vitamin C in tablet is 565 mg

          = 565 mg