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Past paper help - HSN forum

# Past paper help

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### #1will_789

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Posted 25 May 2006 - 10:39 AM

Could someone show me how to do this please?

A mixture of magnesium bromide and magnesium sulphate is know to contain 3mol of magnesium and 4mol bromide ions.
How many mole of sulphate ions are present?

My first stage was to write out the formula for both magnesium bromide and magnesium sulphate.
MgBr(small2) and MgSO(small4)

So if there are 4 moles of br, there are 2 moles of mg for MgBr(small 2).
Leaving 1mol of Mg.
Therefore MgSO(small4) contains 1mol of Mg ions, therefore there are 4 moles of SO(small4) ions.....but thats wrong....what have i done wrong???

### #2d00pyd

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Posted 25 May 2006 - 11:01 AM

QUOTE(will_789 @ May 25 2006, 11:39 AM)

Could someone show me how to do this please?

A mixture of magnesium bromide and magnesium sulphate is know to contain 3mol of magnesium and 4mol bromide ions.
How many mole of sulphate ions are present?

My first stage was to write out the formula for both magnesium bromide and magnesium sulphate.
MgBr(small2) and MgSO(small4)

So if there are 4 moles of br, there are 2 moles of mg for MgBr(small 2).
Leaving 1mol of Mg.
Therefore MgSO(small4) contains 1mol of Mg ions, therefore there are 4 moles of SO(small4) ions.....but thats wrong....what have i done wrong???

I think that you are nearly there. In MgSO4 there is only one mole of So4. The 4 refers to the number of oxgen atoms in so4. If there was 4 moles of SO4 it would need to be (So4)4. Does that makes sense? Therefore is the answer 1?

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Posted 25 May 2006 - 11:03 AM

I get that there is only one mole of SO(small 4) ions because the 4 is only for the oxygen and not for the whole sulphate ion. You might find it helps to put the sulphate ion in brackets so that you can see how many there are.

EDIT: Sorry I just posted at the same time as the person above so I am basically just repeating what they are saying.

### #4George =D

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Posted 25 May 2006 - 11:05 AM

Because there are two Bromide ions for every one Mg ion (ie MgBr2), and there are 4 bromide ions present, all the ions must be present in this compound.

Since 1 mole of ions of Mg : 2 moles of ions of Br, 2Mg : 4Br

So that's 2 of the moles of Mg ions taken care of, therefore the other 1 mole of ions of Mg must be in the MgS04 solution.

and because 1Mg:1SO4, there must be 1 mole of ions of SO4.

Btw, the 4 on the SO4 is NOT the valency of Sulphate. From the data booklet, the valency of SO4 is 2-.

If all else fails, pop into your school and have a word with your Chemistry teacher.

*Edit* - Damn, beaten to the post!

### #5will_789

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Posted 25 May 2006 - 11:12 AM

Thanks folks, im not really wanting to loose easy marks in this paper, thanks!

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