Question

Calculate the concentration of an iodate solution prepared by dissolving 1.9853 g of KIO₃ and diluting to 500 mL with distilled water in a volumetric flask.

A 25 mL aliquot of a 0.0195 M KIO₃ solution is added to a flask containing 2 g of KI and 10 mL of 0.5 M H₂SO₄. The resulting solution is titrated to a starch endpoint with 34.81 mL of the thiosulfate solution. Calculate the concentration of the thiosulfate solution.

How will you know when the endpoint of the titration is reached?

Part B:

Identify the titrant, the primary standard, and the actual chemical species being titrated.

Write the reaction for the formation of the species titrated.

Write the reaction for the multi-step reaction sequence used to analyze Vitamin C.

In this experiment, ascorbic acid is oxidized to dehydroascorbic acid. What changes in the structure over the course of the oxidation suggest an oxidation has taken place?

Answer 1

1.

Calculate the concentration of an iodate solution prepared by dissolving 1.9853 g of KIO3 and diluting to 500- mL withdistilled water in a volumetric flask.

Ans) 1.9853 g x (1mol/ 214.001 g)

= .0.009277059 mol / .500 L = 0.018554 M

2.

A 25- mL aliquot of a 0.0195 M KIO3 solution is added to a flask containing 2 g of KI and 10 mL of 0.5 M H2SO4. The‐resulting solution is titrated to a starch endpoint with 34.81 mL of the thiosulfate solution. Calculate the concentration of the thiosulfate solution.

Ans). Equations:

1) IO3-+8 I-+6 H+ >> 3 I3-+3 H20

2 ) 3 I2+6 S2O3- --> 6 I-+3S4O6-

Given: 25.00 ml of 0.0195 M IO3- and 34.81 ml thiosulfate0.02500 L x 0.0195 mol/L IO3 -

= 4.88E-4 mol IO3 -4.88E-4 mol IO3- * 3 mol I 3-/1 mol IO 3- * 2 mol S2O3 2-/1 mol I 3-

= 0.00293 mol S2O3 2-(2.93E-3 mol S2O3 2-)/(0.03481L)= 0.0842 M S2O3 2-

3) How will you know when the endpoint of the titration is reached?

Ans) Once the starch indicator changes from blue-black to colorless.

PartB:

4.

Identify the titrant, the primary standard, and the actual chemical species being titrated.

Titrant: thiosulfate

Primary standard: iodate

actual chemical species being titrated: ascorbic acid(x)(triiodide?)<---- We’re doing a back titration