posted by mimi--PLEASE HELP VERY CONFUSED .
Titration of Vitamin C tablets
Prior to titration, Vitamin C samples were dissolved in dilute sulfuric acid, treated with a 60mL portion of 0.3 M KIO3 and excess KI.
(1) ascorbic acid + I2 + 2H2O <--> dehydroascorbic acid + 2I^- + 2H3O^+
(2) IO3^- + 5I^- + 6H3O^+ --> 3I2 + 9H2O
(3)I2 + 2S2O3^2- --> 2I^- + S4O6^2-
a) Thiosulfate consumed(mmol)
b) Excess I2 (mmol)
c) Ascorbic acid content (mmol)
d) Ascorbic acid conten (mg)
I am unsure how to go about finding these results -- i would think i would use a mol ratio from the given equations, but im confused with what equations to use ... any help would be GREATLY appreciated!
a. Thiosulfate consumed. Remember that molarity = moles/L OR mmoles/mL. So mmoles thiosulfate = M x mL. I assume you have the molarity and the mL of thiosulfate used.
b. The excess I2 is what was titrated with the thiosulfate in question a. So mmoles I2 = just 1/2 mmoles thiosulfate. You find that from the mole ratio in the I2/S2O3 equation.
c. How much I2 was present to react with vitamin C? That is 60 mL IO3^- x 0.3 M = 18 mmoles IO3^-. That produced 18 x 6 = 108 mmoles I or 54 mmoles of I2 (equation 2). Now subtract from that the excess I2 present from part b. That will give you the I2 that reacted with vitamin C. And since 1 mole vitamin C is equivalent to 1 mole I2 (equation 1), that will also be the mmoles of vitamin C.
d.You should be able to convert from mmols to mg.
Check my thinking.
thank you so much for your help!