Redox titration questions are very popular on exam papers. They are usually set in an unfamiliar context and may require you to work through a number of steps back through an experiment to get to the answer. The key is, as always, to work logically from what you know. Annotate your working as you go so that you don’t lose your way.
The video works though a typical multi-step redox titration question, giving you a fool proof method to tackle these calculations with confidence.
- The amount of iron in an iron/vanadium alloy was found by the following method:
- 0.175g of the alloy wire and 100cm3 of 2.0 mol dm-3 sulphuric acid were added to a conical flask and warmed.
- The flask was cooled and the contents titrated against 0.0216 mol dm-3 potassium dichromate(VI). An average titre of 23.50cm3 was recorded.
6Fe2+ + Cr2O72– + 14H+ ⇾ 6Fe3+ + 2Cr3+ + 7H2O
Determine the percentage by mass of iron in the alloy.
- The concentration of an ethanedioate solution can be found by titration with acidified potassium manganate(VII).
5C2O42– + 2MnO4– + 16H+ ⇾ 10CO2 + 2Mn2+ + 8H2O
A student dissolved potassium ethanedioate-1-water crystals in 100cm3 water. 25.0cm3 of this solution reacted with 18.40cm3 of 0.0500 mol dm-3 potassium manganate(VII) solution. Calculate the mass of K2C2O4•H2O used to make up the original solution.Â
- E300 is an antioxidant used in wines. The maximum concentration of E300 in drinks is 150 mg dm-3. A student took 250.0cm3 of wine and added 25.0cm3 of 0.00500mol dm-3 potassium iodate, followed by excess potassium iodide solution.Â
IO3– + 5I– + 6H+ ⇾ 3I2 + 3H2O
Some of the iodine formed reacted with the E300 in the wine.
C6H8O6 + I2 ⇾ C6H6O6 + 2H+ + 2I–
The amount of unreacted iodine was found by titrating with 0.00500 mol dm-3 sodium thiosulphate using a starch indicator. A titre of 20.40cm3 was recorded.
I2 + 2S2O32– ⇾ 2I– + S4O62–
Determine the concentration of E300 in the wine and hence whether within the limit allowed.
Answers
