Volumetric analysis (2005 VCE)

For quality control, a chemist analyses the vitamin C (molecular formula C6H8O6) content of a new brand of fruit juice. The reaction used is an oxidation-reduction reaction in which I3- is the oxidant and vitamin C is the
The reaction is

C6H8O6(aq) + I3- (aq) →C6H6O6(aq) + 2H+(aq) + 3I-–(aq)

The half reaction for the oxidation of I-–(aq) to I3- (aq) is
I3- (aq) + 2e– → 3I-–(aq)

Give the half reaction for the oxidation of vitamin C.

A 20.00 mL sample of the fruit juice was made up to 250.0 mL with pure water in a volumetric flask.
25.00 mL aliquots of the diluted fruit juice were then placed in a conical flask and titrated with a solution
in which [I3-–] = 2.00 X 10-–4 M.

An average titre of 15.65 mL was obtained.
Calculate the amount of I3- present in the average titre, in mole.

Calculate the amount of vitamin C present in each 25.00 mL aliquot, in mole.

Calculate the concentration of vitamin C in the original (undiluted) sample of fruit juice in mole per


During the analysis the chemist rinsed, but did not dry, each item of glassware. She had available for use:
pure water, the original fruit juice, the diluted fruit juice and the standard
I3- solution. For each item listed below, name the liquid that should be used to rinse it by placing a tick in the appropriate box.