Question about redox

The percentage purity of a sample of manganese(IV) oxide, MnO₂ can be determined by its reaction with acidified iron (II) ions.

Stage 1 - A sample of known mass of the impure MnO₂ is added to a conical flask

Stage 2 - The sample is reacted with a known excess amount of Fe²⁺ acidified with dilute sulphuric acid

Stage 3 - The contents of the flask are heated gently

Stage 4 - The cooled contents of the flask are titrated with aqueous potassium manganate (VII) in acidic conditions to find the amount of unreacted Fe²⁺

i) The reduction half-equation for manganese (IV) oxide in the presence of dilute acid is shown below:

MnO₂ (s) + 4H+ (aq) + 2e^- -> Mn²⁺ (aq) + 2H₂O(l)

Construct the balanced equation for the redox reaction between Fe²⁺ (aq), MnO₂ (s) and H+ (aq)

[1 mark]

ii) In stage 1 and stage 2 a student uses a 0.504g sample of impure MnO₂ and 100cm³ of 0.2moldm^-3 Fe²⁺
In stage 4 the student determines that the amount of unreacted Fe²⁺ is 0.0123 mol.

1 mol of MnO2 reacts with 2 mol of Fe²⁺
Calculate the percentage purity of the impure sample of MnO₂.
[3 marks]

Hi there,
I'm just doing an F325 question for homework and I'm a bit confused about constructing balanced equations. I'm pretty sure I'm missing something stupid but my teachers haven't been great in explaining it.

It's the same question queried here: http://www.thestudentroom.co.uk/showthread.php?t=1671053

I mostly understand tomo511's explanation, but how are you supposed to know Fe²⁺ is oxidised to Fe³⁺? Are those the only states in which an Fe ion can exist? You need to know this so you know to double the Iron(II) side of the equation before combining them.

I feel your pain.

The transition metals have variable oxidation states (up to 2+n for 4s2 3dn) and it's not always clear which ones are stable.

The only real answer is to learn the common oxidation states (brackets - also, but not common).

Ti: 2, (3), 4
V: 2, 3, 4, 5
Cr: 2, 3, 6
Mn: 2, (3), 4, (6), 7
Fe: 2, 3, (6)
Co: 2, 3
Ni: 2, 3
Cu: 1, 2

and a few of the common transformations in redox ...