Question

1).An aluminium oxalate complex was prepared by dissolving aluminium metal in potassium hydroxide, followed by reaction with oxalic acid.The resulting complex was analysed for oxalate content by titration against potassium permanganate:

A 0.1105 g sample of the aluminium oxalate complex was dissolved in deionized water (30 cm³) and bench dilute (2 M) sulphuric acid (10cm³)added. The resulting solution was heated to about 60 ºC and titrated against standardized potassium manganate (VII) solution.14.35 cm³ of a 0.02 mol dm^-3 KMn0₄ solution was required.

(i) Write out a balanced equation for the overall redox process.( 3 marks

(ii) Use the analytical data to deduce if the prepared complex was K₃Al(C₂O₄)₃.3H₂O or KAl(C₂O₄)₂(H₂O)₂.2H₂O. .

                        Molar masses /g mol^-1

                        K, 39.00; Al, 26.98; C, 12.01; O, 16.00; H, 1.0

Answer 1

potassium tris(oxalato)aluminate(III)

Aluminium will react with KOH to give aluminium hydroxide

When it reacts with oxalic acid it will form the aluminium oxalate complex

(i) the overall redox reaction will be

oxalate ion get oxidized to CO2

And KMnO4 gets reduced to Mn⁺² ( from +7 to +2)

2MnO₄^- + 5H₂C₂O₄ + 6H⁺ =>10 CO₂ + 2 Mn²⁺ + 8H₂O

(ii) A 0.1105 g sample of the aluminium oxalate complex was dissolved in deionized water (30 cm³) and bench dilute (2 M) sulphuric acid (10cm³)added. The resulting solution was heated to about 60 ºC and titrated against standardized potassium manganate (VII) solution.14.35 cm³ of a 0.02 mol dm^-3 KMn0₄ solution was required.

The amount of complex dissolved = 0.1105 grams

Volume = 30mL

The Moles of KMnO4 used = Molarity X volume = 0.02 X 14.35 = 0.287 millimoles

Millimoles of oxalate reacted = 5 X 0.287 / 2 = 0.717 millimoles

So mass of oxalate reacted = Moles X molecular weight

Mass = 0.717 X 88 = 0.063 grams

these grams = x% of 0.1105 grams

so x % = 57%

In complex

K₃Al(C₂O₄)₃.3H₂O

462 = molecular weight  

oxalate in this is = 264 grams

Which is 57% of the given complex

So the complex should be K₃Al(C₂O₄)₃.3H₂O