Question

Vitamin C ascorbic acid (C6F1806), is a reducing agent. One way to determine the ascorbic acid content of a sample is to mix the acid with an excess of iodine

C6H8O6 + I2 + H2O------- C6H6O6 + 2H3O + 2I-
and then titrate the iodine that did not react with the ascorbic acid with sodium thiosulfate. The balanced, net ionic equation for the reaction occurring in this titration is
I2+ 2 S2O32-(a9) —) 2 I-(a9) + S4O62-(a9)
Suppose 50.00 mL of 0.0520 M 12 was added to the sample containing ascorbic acid. After the ascorbic acid/I2 reaction was complete, the 12 not used in this reaction required 20.30 mL of 0.196 M Na2S2O3 for titration to the equivalence point. Calculate the mass of ascorbic acid in the unknown sample.

Answer 1

The mass of ascorbic acid in the sample is 0.1076g

Step-by-step explanation:

In other words to determine ascorbic acid in a sample:

You are adding an amount of I₂. One part is reacting with ascorbic acid and the other with sodium thiosulphate. First, we need to calculate how much thiosulphate reacts to know the iodine that reacts with thiosulphate.

I₂ that reacts with S₂O₃²⁻:

Moles S₂O₃⁻: 0.02030L × (0.196mol / L) = 3.979x10⁻³ moles S₂O₃²⁻.

As 2 moles of S₂O₃²⁻ reacts per moles of I₂, moles of I₂ are:

3.979x10⁻³ moles S₂O₃²⁻ ₓ (1 mole I₂ / 2 moles S₂O₃²⁻) = 1.989x10⁻³ moles I₂

Now, initial amount of I₂ added to the sample is:

0.0500L ₓ (0.0520mol I₂ / L) = 2.600x10⁻³ moles I₂

That means moles of I₂ that react with ascorbic acid = Moles of ascorbic acid are:

2.600x10⁻³ moles I₂ - 1.989x10⁻³ moles I₂ =

6.11x10⁻⁴ moles I₂ = Moles C₆H₈O₆.

To convert moles of substance to mass we need to use molar mass of the substance (176.12g/mol for ascorbic acid):

6.11x10⁻⁴ moles C₆H₈O₆ ₓ (176.12g / mol) =

The mass of ascorbic acid in the sample is 0.1076g