Homework Help: chemistry

Posted by mimi--PLEASE HELP VERY CONFUSED on Monday, November 23, 2009 at 10:08pm.

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.

Given equations:

(1) ascorbic acid + I2 + 2H2O <--> dehydroascorbic acid + 2I^- + 2H3O^+

(2) IO3^- + 5I^- + 6H3O^+ --> 3I2 + 9H2O

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

To find:

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!

chemistry - DrBob222, Monday, November 23, 2009 at 10:39pm
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.
chemistry - mimi--PLEASE HELP VERY CONFUSED, Tuesday, November 24, 2009 at 12:01am
thank you so much for your help!

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