How do you work this out? :/

[crayolé]

Selenium dioxide (Se02) is an important reagent in organic syntheses, as it is both an oxidant and
weakly acidic. In a certain reaction, 0.142 g of selenium dioxide reacted with exactly 25.52 mL of
0.100 M chromium(II) sulfate, CrS04. In the reaction, the Cr2+(aq) ions were oxidised to Cr3+(aq).
To what oxidation state was the Se4+ converted in this reaction?

[stonecold]

I posted this somewhere but can't remember where now.

Basically, work out the mol for each.  It ends up being Cr:Se is 2:1

Then write up equation:

2Cr²⁺ + Se⁴⁺ ---> 2Cr³⁺ + Se^x

x must equal + 2 for the charges to balance.

[kate26]

was this in a neap exam ? only cos i remember reading this and thinking wtf?

[stonecold]

yeah, i got it in like the final 3 minutes haha...

[Martoman]

NEAP 09 first short answer

[cameron_15]

Relatively hard exam that was.

[kate26]

haha stupid neap

[cindyy]

I posted this somewhere but can't remember where now.

Basically, work out the mol for each.  It ends up being Cr:Se is 2:1

Then write up equation:

2Cr²⁺ + Se⁴⁺ ---> 2Cr³⁺ + Se^x

x must equal + 2 for the charges to balance.

i dont get how you did that :S

[stonecold]

Did you work out the mol?

I don't exactly remember, but it was something like n(Cr)=0.0028 and n(SeO2)=0.0014.

So from this, you know the mol ratio.  You also know Cr goes from +2 to +3, and the Se is initially +4.

Then do my little equation thingy, remembering to take into account the mol ratio, and you will see the Se has to be reduced to the +2 oxidation state for the charges to balance.

[azn_dj]

Basically because you are not tweaking the balancing by adding electrons/hydrogens to either side
the left side overall charge must equal the right hand side charge
so the left is 2*+2 plus +4 which is +8 Overall
Right hand side - 2*+3 (Chromium) is +6. The difference is 2. So it ends up being 2+ for the Selenium.

Oxidation number in Stonecold's example - Very funny Stonecold

[cindyy]

OHHHHHHH! stupid me