Chemistry 3/4 2013 Thread

[Stick]

Yes, I need to revise for my gravimetric analysis SAC! I'm counting on you to ask questions!!! XD

[teletubbies_95]

I was revising gravimetric analysis and I came across this question. I tried it 5 times, but I dont know how to get it.

Iron, in the form of ‘dried’ iron(II) sulfate, is sometimes prescribed by doctors for patients suffering from anaemia and for women who are pregnant. To determine the iron content of some such iron tablets, 10 tablets were carefully crushed and then weighed. Their mass was 3.345 g. These tablets were then dissolved in water and the insoluble residue filtered off. Potassium dichromate was added to oxidise all the Fe2
present to Fe3
. Next, sodium hydroxide was added until no more iron(III) hydroxide was observed to form. The mixture was then boiled to help granulate the precipitate, which was subsequently collected by filtration. The equation for this reaction is:
Fe3
(aq)
3OH(aq) Fe(OH)(s)

Finally the filter paper and precipitate were heated in a crucible to form iron(III) oxide and to destroy the filter paper.
2Fe(OH)3(s) Fe2O3(s)
3H2O(g)

The mass of iron(III) oxide was found to be 1.42 g.
It is now possible to calculate:
(a) the mass of iron per tablet

Thanks

[Stick]

That is a great question! If I'm on the correct path here, they've basically thrown a hell of a lot of unnecessary information to try and get you confused. I'll see if I can do it.

m(Fe2O3)=1.42g
n(Fe2O3)=m/M=1.42/159.8=0.00888610763mol
n(Fe)=0.01777221526mol
m(Fe)=n*M=0.01777221526*55.9=0.99346683303g (10 tablets)
m(Fe)=0.0993g (1 tablet)

[teletubbies_95]

Omg, thanks so much stick. Great explanation btw. but the answer for some reason is 0. 3345g for one tablet. Im not sure how but?

[Limista]

That is a great question! If I'm on the correct path here, they've basically thrown a hell of a lot of unnecessary information to try and get you confused. I'll see if I can do it.

m(Fe2O3)=1.42g
n(Fe2O3)=m/M=1.42/159.8=0.00888610763mol
n(Fe)=0.01777221526mol
m(Fe)=n*M=0.01777221526*55.9=0.99346683303g (10 tablets)
m(Fe)=0.0993g (1 tablet)

 I agree with everything you've done, but shouldn't you divide n(Fe) in 10 tablets by 10, instead of dividing the mass by 10? Or doesn't it matter? 

[Stick]

Hmm... that's weird. Basically, I've assumed that all of the iron in the iron sulfate has made its way to the final product. I'm very interested to see how you get that answer.

Starfish: you get the same answer either way.

[SeanLee95]

Looks like VCAA has a broken link on their website, the above one doesn't work for me either.

Does this one work? http://www.vcaa.vic.edu.au/Documents/vce/chemistry/Chemistryexamsample.pdf

can u please link us the answers for the sample exam aswell?.. thanks!

[thushan]

They didn't write answers. Good point actually; I should get onto writing the official AN solutions to the exam!

[Limista]

Starfish: you get the same answer either way.

k Thanks.

[KevinooBz]

I was revising gravimetric analysis and I came across this question. I tried it 5 times, but I dont know how to get it.

Iron, in the form of ‘dried’ iron(II) sulfate, is sometimes prescribed by doctors for patients suffering from anaemia and for women who are pregnant. To determine the iron content of some such iron tablets, 10 tablets were carefully crushed and then weighed. Their mass was 3.345 g. These tablets were then dissolved in water and the insoluble residue filtered off. Potassium dichromate was added to oxidise all the Fe2
present to Fe3
. Next, sodium hydroxide was added until no more iron(III) hydroxide was observed to form. The mixture was then boiled to help granulate the precipitate, which was subsequently collected by filtration. The equation for this reaction is:
Fe3
(aq)
3OH(aq) Fe(OH)(s)

Finally the filter paper and precipitate were heated in a crucible to form iron(III) oxide and to destroy the filter paper.
2Fe(OH)3(s) Fe2O3(s)
3H2O(g)

The mass of iron(III) oxide was found to be 1.42 g.
It is now possible to calculate:
(a) the mass of iron per tablet

Thanks

3.345/10=0.3345

[Stick]

WTF? Question was written wrong! HAHAHAHAHA!!!!!

Good pick-up, KevinooBz.

[Limista]

WTF? Question was written wrong! HAHAHAHAHA!!!!!

Good pick-up, KevinooBz.

3.345/10=0.3345

I also noticed what KevinooBz wrote. But why does it mean that the question was written wrong?

I'm confused. 

[Stick]

Well if that solution was true, we're assuming each tablet consists of nothing else but iron. Yet it says that the tablet is made from iron sulfate. Obviously, that isn't possible. You'd also expect some other substances in the tablet as well (it said some residue didn't dissolve).

Keep the gravimetric analysis questions coming. Help for you results in help for me!

[zvezda]

Hang on, unless I'm doing something reaaallllly wrong and stupid, wouldnt you just find the amount of mol of FeSO4 in the 3.345g, divide that by 10 and then multiply by the molar mass of Fe???

[Stick]

Again, this confusion has probably arisen from the question. We're assuming that there are other substances in that tablet as well, since it says some residue didn't dissolve. Otherwise, if the entire tablet was iron sulfate, there would be no residue.

However, if the entire tablet was composed of only iron sulfate, your method is correct.