The concentration of Fe2+ ions in a solution can be determined by titration with acidifed potassium manganate(VII) solution.
The ionic equation for the reaction is:
5Fe2+ + MnO4– + 8H+ → 5Fe3+ + Mn2+ + 4H 2O
A 25.0 cm3 sample of solution containing Fe2+ ions reacted exactly with 10.0 cm3 of 0.0500 mol dm–3 potassium manganate(VII) solution.
What is the concentration of Fe2+ , in mol dm–3 ?

Answer:

18) E

We are given the concentration and volume of potassium manganate (VII). We can calculate the number of moles with this.

(convert volume to dm3 first. 10 cm3 = 0.01 dm3)

n = c x v = 0.05 x 0.01 = 5 x 10-4 mol

From the reaction equation, we can see that molar ratio of Fe2+ : MnO4- = 5 : 1

So, 1 mole of MnO4- ions reacts with 5 moles of Fe2+ ions.
So, 5 x 10-4 mol of MnO4- ions would react with (5 x 5 x 10-4) moles of Fe2+ ions.
Moles of Fe2+ = 25 x 10-4 mol

We are given the volume of Fe2+ = 25 cm3 = 0.025 dm3

Concentration = moles ÷ volume