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