[psychkub]

Can someone just confirm I did this right?

Ascorbic Acid (C₆H₈O₆, MM=176.126) acts as a reducing agent as follows
C₆H₈O₆ -> C₆H₆O₆ + 2H⁺ +2e^-
It can be oxidized by I₂ +2e^- -> 2I^-
A 200.0mL sample of a citrus fruit drink is acidified, and 100.0mL of .0500M I₂ is added. All of the ascorbic acid is oxidized and some iodine remains in excess. After the reaction is complete this excess iodine is titrated with 38.62mL of .0120M Na₂S₂O₃. Compute the number of milligrams of ascorbic acid per mL of fruit drink.

So the first thing I did was find the net result of the redox reaction and got:
C₆H₈O₆ + I₂ -> C₆H₆O₆ + 2 I^- + 2H⁺

Next I found the total amount of iodine that reacted in both the redox part and in the titration and got:
.0500mol I₂/L * .01000L = 5.00e-4 mol I₂

Next, I found the amount of Na₂S₂O₃ that would react with the iodine and  used that amount to stoichiometrically find the amount of iodine that was used for the titration and got:
.0120 mol Na₂S₂O₃/L * .03862L = 4.6344e-4 mol Na₂S₂O₃
which is also the amount of S₂O₃ available due to the one to one relationship.

Since I₂+ 2S₂O₃ -> 2I^-+ S₄O₆^2-
4.6344e-4 mol S₂O₃ * mol I₂/2 mol S₂O₃ = 2.3172e-4 mol I₂

To find the amount of iodine that initially reacts:
5.00e-4 - 2.317e-4 = 2.6828e-4 mol I₂

To find the mg of ascorbic acid:
2.6828e-4 mol ₂ * mol C₆H₈O₆/ mol I₂ = 2.6828e-4 mol C₆H₈O₆

2.6828e-4 mol C₆H₈O₆ * 176.126g/mol C₆H₈O₆ = .047251g

47.251mg/200.0mL = .2363mg/mL

is that right?

[Astrokel]

hey psychkub,

Brief through it, your steps are all correct, just remember to put the charge of thiosulphate in your equations.