MnO4– is a strong oxidising agent and so it be used in titrations to determine the amount of a reducing agent present.
MnO4– has a very intense purple colour and is reduced to Mn2+ which is a very pale pink and almost colourless at low concentrations. In a typical titration, MnO4– is added from the burette and is quickly reduced to Mn2+ (i.e., the purple colour disappears almost instantaneously). At the end-point, the supply of reducing agent is exhausted so the MnO4– is no longer reduced and this can be seen by the permanent apperance of a pale pink / purple colour in the mixture. This means that MnO4– is self-indicating.
| Question: 6.96 g of FeCl2.xH2O is dissolved in distilled water to form 250 cm3 of solution. A 25 cm3 sample of this solution is titrated with 35.0 cm3 of 0.02 mol/dm3 acidified KMnO4. Determine the value of x. Step 1 We know that moles = concentration x volume: n MnO4– = 0.02 x (35 / 1000) = 7.00 x 10-4 mol Step 2 The balanced symbol equation isn’t given so we need to work it out. You should be able to see that this is a redox reaction with Fe2+ oxidised to Fe3+ and MnO4– reduced to Mn2+. Write out each of the half-equations seperately and then balance in terms of the central ion, O, H and charge. We then need to combine the reactions by multiplying the oxidation reaction by 5, and adding both sides: Oxidation Fe2+ –> Fe3+ + e– Reduction MnO4– + 8 H+ + 5e– –> Mn2+ + 4 H2O Total 5 Fe2+ + MnO4– + 8 H+ + 5e– –> 5 Fe3+ + Mn2+ + 4 H2O Step 3 The overall balanced symbol equation tells us that Fe2+ (i.e., FeCl2.xH2O) reacts with MnO4– (i.e., KMnO4) in a 5:1 ratio. n FeCl2.xH2O = 5 x 7.00 x 10-4 = 0.0035 mol Step 4 The total is 10x bigger than the sample (i.e., 250 / 25): n FeCl2.xH2O = 10 x 0.0035 mol = 0.035 mol in total Step 5 We have the mass of FeCl2.xH2O (6.96 g) and can assume that it is pure. This means that the Mr of FeCl2.xH2O is: Mr FeCl2.xH2O = 6.96 / 0.035 = 198.8 Step 6 Finally, we subtract the Mr of FeCl2 (126.8) to obtain the Mr of the xH2O part of the formula Mr xH2O = 198.8 – 126.8 = 72 The Mr of H2O is 18, so clearly x = 4 |