Thread: Help with this chemistry question!

An iron nail was dissolved in excess dilute sulphuric acid to produce 100 cm3 solution. A sample of 10 cm3 of this solution required 4 x 10-4 mol of potassium manganate (VII) for complete reaction.


Fe(s) + H2SO4 (aq) Fe2SO4 (aq) + H2 (g)
5Fe2+ (aq) + MnO4- Mn2+ + 4H2O (l) + 5Fe3+ (aq)

If the mass of nail used was 1.45g, calculate the number of moles of Fe2+ produced in the reaction.

The solution they gave was simple:

5 x (4 x 10-4) x 10
= 0.02 mol.

But I don't understand. If 1 mol of MnO4 requires 5 mol of Fe2+, wouldn't it mean that 1 mol of Fe 2+ uses 1/5 mol of MnO4 to produce 1 mol of Fe3+? So, shouldn't it be 4 x 10-4 / 5 instead of 4 x 10-4 x 5?

Also, why do they need to x 10?

My solution was 1.45 / 56 = 0.03 mol since 1 mol of Fe require 1 mol of H2SO4 to produce 1 mol of FeSO4. Why is this not accepted? Thanks.

You multiply by 10 cause you titrated 10cms only of the slution... when there is 100cm

And the logic up there is very simple, 1 mol of km04 reacts with 5Fe2 ? Right, its not asking you how much of km04 1 mol of fe2+ require but rather the amount of Fe2+ used, so if 1 mol of kmo4 was used, it had to react with 5 mole of fe2+