The following results were obtained when a sample of pineapple juice was analyzed to determine its vitamin C content. A 19.8 mL sample of pineapple juice was placed in a flask with 20 ml of 0.5 M H2SO4, 1.0 g of potassium iodide, KI, and 25.00 mL of 0.01988 M potassium iodate, KIO3. Titration with sodium thiosulfate required the use of 23.85 mL of 0.08014 M Na2S2O3. Calculate the concentration of vitamin C in the sample in mg vitamin C per 100 mL juice.

This is an example of a back titration; i.e., the KI/KIO3 produces I2, the I2 reacts with some of the vit C, the excess I2 is titrated with thiosulfate.

6H^+ + I^- + IO3^- ==> 3I2 + 3H2O
Then vitC + I2 ==> 2I^- + vitCx
mols IO3 added initially = M x L = 0.025 x 0.01988 = approx and you must do this more carefully = 0.0005
mols I2 formed initially = 0.0005 x (3 mol I2/1 mol IO3^-) = 0.0015.

How much I2 was left; i.e., the amount left is the amount vit C did NOT react with.
2S2O3^2- + I2 ==> 2I^- + S4O6^2-
mols thiosulate = M x L = 0.08014 x 0.02385 = approx 0.002
Convert 0.002 mols S2O3^2- to mols I2. From the equation that is 0.002 x 1/2 = about 0.001 mols I2 not used by the vit C. So the difference between initial and end is amount used by vit C. That is 0.0015-0.001 = approx 0.0005 mols vit C analyzed.
g vit C = mols x molar mass in 19.8 mL sample. Convert to mg/100 mL.

To calculate the concentration of vitamin C in the sample, we can use the iodometric method. In this method, iodine is generated by the reaction between potassium iodate and potassium iodide, and then it reacts with vitamin C to form iodide ions.

Here's how we can calculate the concentration of vitamin C:

1. Calculate the number of moles of iodine produced:
- The balanced equation for the reaction between potassium iodate (KIO3) and potassium iodide (KI) is:
5IO3- + I- + 6H+ → 3I2 + 3H2O
- From the equation, we can see that 5 moles of IO3- react with 1 mole of I- to produce 3 moles of I2.
- The number of moles of I2 can be calculated using the stoichiometry of the reaction:
Moles of I2 = (Volume of KIO3 solution in L) × (Molarity of KIO3 solution)

2. Calculate the number of moles of sodium thiosulfate:
- The balanced equation for the reaction between iodine (I2) and sodium thiosulfate (Na2S2O3) is:
I2 + 2S2O3^2- → 2I- + S4O6^2-
- From the equation, we can see that 1 mole of I2 reacts with 2 moles of Na2S2O3.
- The number of moles of Na2S2O3 can be calculated using the stoichiometry of the reaction:
Moles of Na2S2O3 = (Volume of Na2S2O3 solution in L) × (Molarity of Na2S2O3 solution)

3. Calculate the number of moles of vitamin C:
- From the balanced equation for the reaction between iodine (I2) and vitamin C (C6H8O6), we can see that 1 mole of vitamin C reacts with 1 mole of I2.
- The number of moles of vitamin C can be calculated:
Moles of vitamin C = Moles of I2

4. Calculate the concentration of vitamin C in the sample:
- The molar mass of vitamin C (ascorbic acid) is 176.12 g/mol.
- The volume of the pineapple juice used is 19.8 mL.
- Convert the volume of the pineapple juice to liters:
Volume of pineapple juice in L = (Volume of pineapple juice in mL) / 1000
- Calculate the concentration of vitamin C in the sample:
Concentration of vitamin C (mg/100 mL) = (Moles of vitamin C × Molar mass of vitamin C) / (Volume of pineapple juice in L) × 100

Plug in the values from the question and perform the calculations to find the concentration of vitamin C in the sample.