A student sets up a titration to determine the amount of iron(II) sulfate in an iron tablet. They titrate the iron(II) sulfate solution with potassium manganate(VII) solution.
i)
Write the balanced, ionic half equations to show the reduction of the manganate(VII) ion and the oxidation of the Fe2+.
[2]
ii)
Use your answers to part (i) to write an overall redox equation for the titration of iron(II) sulfate with potassium manganate(VII) solution.
The iron(II) sulfate solution is acidified before titration to stop the manganate ion forming unwanted manganese dioxide. Explain the effect that not acidifying the iron(II) sulfate would have on the final calculation of the estimated mass of iron.
The student dissolved the iron tablet in excess sulfuric acid and made the solution up to 250 cm3 in a volumetric flask. 25.0 cm3 of this solution was titrated with 0.0100 mol dm-3 potassium manganate(VII) solution. The average titre was found to be 26.65 cm3 of potassium manganate(VII) solution.
i)
Calculate the amount, in moles, of iron(II) ions in the 250 cm3 solution.
A voltaic cell is made from a half-cell containing a magnesium electrode in a solution of magnesium nitrate and a half-cell containing a silver electrode in a solution of silver(I) nitrate.
i)
Use section 25 of the data booklet to determine which electrode is positive and to write the equation for the reaction at the positive electrode, including state symbols.
[1]
ii)
Compare the processes at the positive electrodes in voltaic and electrolytic cells.
A 150.0 cm3 sample of pond water was analysed using the Winkler method to determine its biological oxygen demand (BOD). Initially it took 29.40 cm3 of 0.010 mol dm-3 Na2S2O3 to react with iodine.
After five days it required 13.70 cm3 of 0.010 mol dm-3 Na2S2O3 to react with iodine.
The unbalanced equations for the Winker method are shown below.
15.00 cm3 of ethanedioic acid, H2C2O4 (aq), requires 10.30 cm3 of a 0.250 mol dm-3 solution of sodium hydroxide, NaOH (aq), for complete neutralisation using a phenolphthalein indicator for the first permanent colour change.
15.00 cm3of the same H2C2O4 solution required 12.35 cm3 of potassium permanganate solution, KMnO4 (aq), solution for complete oxidation to carbon dioxide and water in the presence of dilute sulfuric acid to further acidify the H2C2O4solution for the first permanent colour change.
i)
Using the following equation, calculate the amount, in moles, of H2C2O4 (aq).
H2C2O4 (aq) + 2NaOH (aq) → Na2C2O4 (aq)+ 2H2O (l)
[2]
ii)
Calculate the concentration, in mol dm-3, of H2C2O4 (aq).
