Ascorbic acid in vitamin c tablet

Question

We are doing a lab about vitamin C and titration of it. I have questions regarding this but the second question threw me off by asking for the moles of ascorbic acid in the erlenmeyer flask.

What am stuck on is the mole ratio. I know I have to use to find the answer by how can I do that with the following balanced equations.

C6H8O6 + I3- + H2O --> C6H6O6 + 3I- + 2H+

and

IO3- + 5I- + 6H+ ---> 3I2 + 3H2O

Answer 1

We are doing a lab about vitamin C and titration of it. I have questions regarding this but the second question threw me off by asking for the moles of ascorbic acid in the Erlenmeyer flask.

What am stuck on is the mole ratio. I know I have to use to find the answer by how can I do that with the following balanced equations.

C6H8O6 + I3- + H2O --> C6H6O6 + 3I- + 2H+ (1)

and

IO3- + 5I- + 6H+ ---> 3I2 + 3H2O (2)

First note that the first half-reaction is not balanced. The water on the left side is the solvent (in excess) and is not part of the reactants. You could add water to the right side, but that would be redundant.

However, water IS produced in the second half-reaction.

The "oxidation" of ascorbic acid is given by the half-reaction

C₆H₈O₆ + I₃^- ===> C₆H₆O₆ +3 I^- + 2 H⁺ (3)

(Note that, in practice, we use an iodide salt, KI, to help solubilize the molecular iodine. The iodide and iodine form the more soluble triiodide complex: I₂ + I^- ==> I₃^- )

The reduction half-reaction for iodate is correct, and produces water as a side product.

IO₃^- + 5 I^- + 6 H⁺ ===> 3 I₂ + 3 H2O (2)

Combining the two half-reactions (2) and (3) gives

C₆H₈O₆ + I₃^- + IO₃^- + 5 I^- + 6 H⁺ ===> C₆H₆O₆ +3 I^- + 2 H⁺ + 3 I₂ + 3 H2O (4)

Simplifying equ (4) by cancelling H⁺ and I^- in both reactants and products gives a net reaction expression.

(Remember that I₃^- = I₂ + I^- )

C₆H₈O₆ + ( I₂ + I^- ) + IO₃^- + 5 I^- + 6 H⁺ ===> C₆H₆O₆ +3 I^- + 2 H⁺ + 3 I₂ + 3 H2O

C₆H₈O₆ + I₂ + IO₃^- + 6 I^- + 6 H⁺ ===> C₆H₆O₆ +3 I^- + 2 H⁺ + 3 I₂ + 3 H2O

C₆H₈O₆ + IO₃^- + 3 I^- + 4 H⁺ ===> C₆H₆O₆ + 2 I₂ + 3 H2O (5)

This balanced net reaction shows that 1 mole of ascorbic acid is oxidized per 1 mole of iodate ion. Therefore, the mole ratio of these reactants is

1 mole C₆H₈O₆ / 1 mole IO₃^-

Also note that the reactant ratio for iodide is

1 mole C₆H₈O₆ / 3 moles I^-

Answer 2

Question by Ghaith A.: Confusion regarding the mole ratio in a lab involving the titration of Vitamin C (ascorbic acid).

Answer:

In your lab, you're working with the titration of ascorbic acid (Vitamin C, chemical formula C6H8O6) and you need to find the moles of ascorbic acid in the Erlenmeyer flask. The key here is understanding the mole ratio from the balanced chemical equations you've provided.

1. The Reaction of Ascorbic Acid with Iodine (I3⁻):
2. C6H8O6 + I3⁻ + H2O → C6H6O6 + 3I⁻ + 2H⁺
3. In this reaction, ascorbic acid (C6H8O6) is reacting with triiodide ion (I3⁻).
4. The Reaction of Iodate Ion (IO3⁻) with Iodide Ion (I⁻):
5. IO3⁻ + 5I⁻ + 6H⁺ → 3I2 + 3H2O
6. This is a standard reaction used in redox titrations involving iodine.
7. Understanding Mole Ratio:
8. From the first equation, each molecule of ascorbic acid reacts with one triiodide ion. This gives you a 1:1 mole ratio between ascorbic acid and I3⁻.
9. In the second equation, each molecule of IO3⁻ produces 3 molecules of I2, which react with I⁻ ions to form I3⁻. Therefore, for each IO3⁻ ion, you effectively get 1 I3⁻ ion in the reaction with ascorbic acid.
10. Finding Moles of Ascorbic Acid:
11. To determine the moles of ascorbic acid in your flask, you need to know the concentration and volume of the titrant used (likely a solution of IO3⁻).
12. Using the stoichiometry of the reactions, you can calculate the moles of ascorbic acid based on the moles of IO3⁻ (or I3⁻) that reacted. Remember to consider the mole ratios from the balanced equations.

Additional Note: It's essential to carefully measure and record the volumes during titration and to use the correct molarity of your titrant. The calculations are straightforward once you understand the stoichiometry of the reactions involved.