1) B. Manganate Redox Titrations

Question 1

What colour is MnO4^-?

Answer 1

Purple

Question 2

Write a method used to determine how much iron is in iron tablets?

Answer 2

1) weigh accurately 2 ‘ferrous sulfate’ tablets.
2) grind the tablets with 1 moldm^-3 sulfuric acidusing a pestle and mortar
3) Through a funnel, transfer the paste into a 100cm^3 volumetric flask. Use 1M sulfuric acid to rinse the ‘ground up’ tablets into the flask
4) add more sulfuric acid until solution is exactly 100cm^3
5) add stopper to flask and shake- not all will be in solution however the Fe^2+ ions in tablet should be dissolved
6) titrate 10cm^3 solution with 0.005M potasssium manganate. Colour change will go from colourless to purple

Question 3

What acid should be used for manganate titrations?

Answer 3

Dilute sulfuric acid.

Question 4

Why shouldn’t you use insufficient volumes of sulfuric acid for manganate titrations?

Answer 4

Solution might not be acidic enough and MnO2 will be produced instead of Mn2^+.
Brown MnO2 will mask the colour change and lead to a greater (inaccurate) volume of manganate being used in the titration.

Question 5

Why cant conc HCl be used?

Answer 5

Cl^- would oxidise to Cl2 .

This would lead to greater volume of manganate being used and poisonous Cl2 being produced

Question 6

Why cant nitric acid be used?

Answer 6

It’s an oxidising agent. Oxidises Fe^2+ to Fe^3+. This would lead to smaller volume of manganate being used

Question 7

What’s the redox equation for the manganate titration?

Answer 7

MnO4^- + 8H^+ +5Fe^2+ ===> Mn^2+ +4H2O +5Fe^3+