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Ok for some reason I just can't work out this question, can someone help please?

A particular sample of bauxite ore contains 55% by mass of Al(2)O(3) (Mr = 102) and no other aluminium compound. The minimum mass of this ore to produce 1 tonne of aluminium is:

A - 1.8 tonne
B - 1.9 tonne
C - 2.9 tonne
D - 3.4 tonne

Thanks v much :smile:

Reply 1

jag_jatt

thats a toughie

sry i tried bt no success

is it frm a past paper? edexcel rite?

Reply 2

kingpong

Hmmmm got my brain working on this one.

The electrolysis of Aluminium is

Al2O3 -> 2Al, therefore 1:2 reaction

number of moles of Al in a tonne = 1x10^6 / 27.0
= 37037 moles

moles Al2O3 needed = 37037 / 2
= 18516 moles

mass Al2O3 in 18516 moles = 18516 x 103
= 1.91x10^6 grams
= 1.91 tonnes

EDIT: Didn't see the % mass part!

= (1.91/55) x 100
= 3.4 tonnes!

(I think) :biggrin:

I this from an AQA multiple choice paper? I vaguely remember it.

EDITTED!

Reply 3

jag_jatt

kingpong

Hmmmm got my brain working on this one.

The electrolysis of Aluminium is

Al2O3 -> 2Al, therefore 1:2 reaction

number of moles of Al in a tonne = 1x10^6 / 27.0
= 37037 moles

moles Al2O3 needed = 37037 / 2
= 18516 moles

mass Al2O3 in 18516 moles = 18516 x 103
= 1.91x10^6 grams
= 1.91 tonnes

(I think) :biggrin:

raaaaaaaaaa

Reply 4

Asphyxiating

OP

2

kingpong

Hmmmm got my brain working on this one.

The electrolysis of Aluminium is

Al2O3 -> 2Al, therefore 1:2 reaction

number of moles of Al in a tonne = 1x10^6 / 27.0
= 37037 moles

moles Al2O3 needed = 37037 / 2
= 18516 moles

mass Al2O3 in 18516 moles = 18516 x 103
= 1.91x10^6 grams
= 1.91 tonnes

(I think) :biggrin:

I this from an AQA multiple choice paper? I vaguely remember it.

Yes it's from AQA multiple choice Jan 2004! My teacher got D - 3.4 tonnes but that could be wrong..she didn't want to explain it though :eek:

However you seem to make sense :smile:

Reply 5

jay9386

the answer is D...i cant remember how i worked it out...ill try find out how i did it

Reply 6

jay9386

moles of Al = 1/27 = 0.0370

2:1 ratio therefore 0.0370 *2 = moles of Al2O3 = 0.074

mass = 0.074 * 102 (RFM of Al2O3) = 7.58g

7.58g * 0.45 = 3.3966

therefore answer is D

If that helps....

Reply 7

Asphyxiating

OP

2

Thank you jay9386 and kingpong that really helps Thanks :biggrin:

Reply 8

kingpong

Asphyxiating

Thank you jag_jett and kingpong that really helps Thanks :biggrin:

Note the edit in my post, I totally missed the % mass part, D is the correct answer!

I HATE AQA MULTIPLE CHOICE.

Reply 9

jay9386

my working is prolly wrong but i got the right answer :confused:

oh well...multiple choice is nasty

Reply 10

Asphyxiating

OP

2

kingpong

Note the edit in my post, I totally missed the % mass part, D is the correct answer!

I HATE AQA MULTIPLE CHOICE.

Yes, thanks I see it but just wondering is the actual electrolysis ratio 2:4 as Al(2)O(3) --> 4Al + 3O(2) , I know 2:4 will be same as 1:2 just checking if this is the actual eq?

Also jay didn't really get your method, but you got the right method! I see your method kingpong, but jay I'm just wondering if you don't have to turn the tonnnes to grams as RMM expressed in g? Anyway doesn't really matter, thanks both of you for your help! :biggrin:

Reply 11

zazy

1

Does anyone have the jan paper and the mark scheme?

Reply 12

kingpong

zazy

Does anyone have the jan paper and the mark scheme?

I have the paper right here :smile:

.

Not on the computer unfortunately though

Here's the markscheme ;p

bdaabccdbcddadadbccdaabdbxaabcadaaccbabc.

EDIT: I'll scan it in in a minute and post it.

Reply 13

kingpong

Asphyxiating

Yes, thanks I see it but just wondering is the actual electrolysis ratio 2:4 as Al(2)O(3) --> 4Al + 3O(2) , I know 2:4 will be same as 1:2 just checking if this is the actual eq?

2Al2O3 + 3C -> 4Al + 3CO2

...is the reaction for the electrolysis of bauxite, so yeah it is 4:2, but only if you include carbon etc which I didn't just for simplicity's sake.

Reply 14

kingpong

I'll send whoever wants the Jan 04 paper on msn.

[email protected]

Reply 15

Asphyxiating

OP

2

Hey again! Ok do you think you could help me quickly on a few questions on this paper? I'm a tad confused. I know it looks long but I just want to check answers for some of them quickly so it won't take you long! :biggrin:

Thanks

14. The equilibrium constant expression for the dissolving of magnesium hydroxide is k = [Mg2+][OH-]^2 . In a saturated soln of Mg(OH)(2) at a different temperature, the concentration of hydroxide ions is 1.0 x 10^(-3) moldm-3

Which one of the following has the correct value and units for K under these conditions?

Apparently it is D - 5 x 10^(-10) mol3dm-9

Ok I get the units but I don't get the value? How do you work it out? I take it conc Mg2+ does not equal conc OH- ? Cos that gets C (1x10^-9)

16. Use your knowledge of chemistry of transition metals to predict which of the following will convert [Mn(H2O)6]2+ into MnO(4)^2-

Apparently it's an alkali and oxidising agent. Ok get the oxidising agent but how do you tell between alkali or acid? If it was an acid would the ligand end up as MnCl(2) or MnSO(4) or something like that instead?

17. Ok well you've got the paper right? So on the diagram, it asks which one of the reagents is not suitable for the step indicated? Well why isn't it HCN because it's toxic? And what's wrong with hot ethanolic KOH? -do you need an acid/H+ instead?

22. Where are the peaks that make up m/z = 144 and 129? I can't get the structures...

31. I get 1,2,3,4 all to be right but apparently 4 is wrong but why is that? Cr goes from 3+ to 3- on both occasions so there is a change in ox.state?

35. m/z = 85 is wrong, right (2)? and 37. m/z = 115 is wrong (2)?

Reply 16

Asphyxiating

OP

2

p.s. your avatar is really distracting kingpong! :tongue:

Reply 17

kingpong

Asphyxiating

p.s. your avatar is really distracting kingpong! :tongue:

Gotta love stewie. :smile:

Okay, I'll try those questions.

Reply 18

kingpong

14. The equilibrium constant expression for the dissolving of magnesium hydroxide is k = [Mg2+][OH-]^2 . In a saturated soln of Mg(OH)(2) at a different temperature, the concentration of hydroxide ions is 1.0 x 10^(-3) moldm-3

Which one of the following has the correct value and units for K under these conditions?

If conc of OH- is 1x10^-3, then

conc Mg2+ = 1x10^-3 / 2
= 5x10^-4

k = [Mg2+][OH-]^2
= 5x10^-4 x (1x10^-3)^2
= 5x10^-10

16. Use your knowledge of chemistry of transition metals to predict which of the following will convert [Mn(H2O)6]2+ into MnO(4)^2-

Apparently it's an alkali and oxidising agent. Ok get the oxidising agent but how do you tell between alkali or acid? If it was an acid would the ligand end up as MnCl(2) or MnSO(4) or something like that instead?

Not sure, but overall H+ ions are being lost in the change so an alkali would be the obvious choice to accept the protons.

Quote:
17. Ok well you've got the paper right? So on the diagram, it asks which one of the reagents is not suitable for the step indicated? Well why isn't it HCN because it's toxic? And what's wrong with hot ethanolic KOH? -do you need an acid/H+ instead?

I think you misinterpreted the question. Basically it's asking you which isn't a reagent in on of the reaction listed. HCN is obviously the reagent for step one, H2SO4 for the dehydration in step 2 and CH3OH/acid for the esterification in step 4.

There aren't any eliminations therefore ethanolic KOH isn't needed.

Quote:
22. Where are the peaks that make up m/z = 144 and 129? I can't get the structures...

A is the incorrect answer because Q doesn't have a C=O bond, the rest doesn't matter, you could spend fricken ages working out all the different m/z values, which should really be a last resort.

31. I get 1,2,3,4 all to be right but apparently 4 is wrong but why is that? Cr goes from 3+ to 3- on both occasions so there is a change in ox.state?

Nope! In X Cr remains in the Cr3+ state, check out the complexes [Cr(H2O)6]3+ --> Cr(OH)3(H2O)3.

Quote:
35. m/z = 85 is wrong, right (2)? and 37. m/z = 115 is wrong (2)?

m/z=85, break the C-O and the left hand side of the ester = 85, C(CH3)3CO.

Reply 19

Asphyxiating

OP

2

On Q14. How can you just find conc Mn(2+) by dividing by 2?

conc Mg2+ = 1x10^-3 / 2
= 5x10^-4

Thanks :tongue:

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