Both the oxalate ligands and the vanadium are oxidised by the KMnO4 with the oxa

Question

Both the oxalate ligands and the vanadium are oxidised by the KMnO4 with the oxalate decomposing and leaving the system as CO2(g). Therefore, the first titration step titrates both the oxalate and the vanadium.

0.3g of the complex salt was dissolved in 60ml of H2SO4 (2M). This solution was diluted to 100ml with distilled water. Take 25ml aliquot and heat the solution to 80 oc. Titrate this hot solution with potassium permanganate 0.01M until the bright pink colour is first seen.

The vanadium must be reduced back down in order to perform a second KMnO4 titration this time with just the vanadium being oxidised as there is no oxalate left in the system.

Add 0.1g of sodium sulphite to the oxidised solution.

The volume of first titration is 16ml and 10.5ml for the second titration.

Calculate the moles of oxalate from the difference between the first and second titrations. (show your working).

Calculate the number of mole of vanadium from the second titration.

Calculate the vanadium: oxalate ratio.

Explanation / Answer

With the first titration, both moles of vanadium and oxalate (total moles) are determined.

The moles of KMnO4 used are equivalent to the moles of oxalate and vanadium.

Thus:

Total n° moles (n° oxalate + n° vanadio ) = 0.025 L x 0.01 moles/L = 2.5 x 10 -4 moles.

With the second titration the moles of vanadium are determined.

The moles of KMnO4 used are equivalent to the moles of vanadium because there is no oxalate.

Thus:

n ° moles de Vanadio = 0.0105 L x 0.01 moles / L = 1.05 x 10 -4 moles

To determine the moles of oxalate, subtract the moles of the vanadium from the total

n ° moles de oxalato = 2.5 x 10 -4 moles - 1.05 x 10 -4 moles of the vanadium = 1.45 x 10-4 moles

Relación de vanadio / oxalato = moles de vanadio / moles totales = 1.05x10 -4/2.5x10-4 = 0.42

Relación de vanadio / oxalato: 0.42: 0.58 = 42:58

n ° moles de Vanadio = 0.0105 L x 0.01 moles / L = 1.05 x 10 -4 moles

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