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# Mole Question Stuck...

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Ok so the question asks, 'calculate the no. of moles of iron in 0.982g of Fe(NH4)2(SO4)2.XH2O

its from a past IB exam paper. If possible, explain what to do instead of just stating the answer.

Merci.

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1. Calculate molar mass of the compound.

2. Divide mass by molar mass to find moles of that compound.

3. Write down number in answer box.

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I am interested in this question as well.

Hold on, this may be wrong and do correct me if it is: With that method, is the number of mole the same for every compound/element for the WHOLE compound? He's only asking for the mole of Fe...

To me, it seems reasonable to do what Drake did, but after step two, you have to multiply that value of the percentage molar mass that iron takes up: MM(Fe)/MM(Fe(NH4)2(SO4)2.XH2O)

Please correct me.

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I am interested in this question as well.

Hold on, this may be wrong and do correct me if it is: With that method, is the number of mole the same for every compound/element for the WHOLE compound? He's only asking for the mole of Fe...

To me, it seems reasonable to do what Drake did, but after step two, you have to multiply that value of the percentage molar mass that iron takes up: MM(Fe)/MM(Fe(NH4)2(SO4)2.XH2O)

Please correct me.

The way I have it would be fine.

Let's assume it's one mole, which I doubt it is.

One mole of that compound=6.02*10^23 molecules.

There is only one iron atom in the molecule so there is only 6.02*10^23 iron atoms.

One mole of iron=6.02*10^23 iron atoms. Now of course this is not exactly one mole, but that only changes the numbers of the ratio but the ratio is still the same.

0.982g of Fe(NH4)2(SO4)2.XH2O

55.8+2(18.04)+2(96.45)=284.78

0.982/284.78=0.00345mol of compound.

What I'm trying to say shows itself at this point. To calculate the number of iron atoms you multiply by Avogadro's but then to change the number of atoms back to moles you divide by Avogadro's so they just cancel out so it's a pointless step.

Last edit: I feel like this question just wants to see if you know to leave the 2 water out of the molar mass of the compound or not Edited by Drake Glau
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I know all the steps, the water molecule was the problem... Do mind telling me why it is omitted?

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Water is not essentially part of the compound.

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yes but doesn't 0.982g of Fe(NH4)2(SO4)2.XH2O include the water?

P.S. sorry for not understanding this, the question seems a little odd to me...

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Is there additional information given?

Actually I did some research and looked back at my old labs. You actually ARE suppose to add the water to the MM. I just don't know how to find X....

If you can find X, it's add 18X to the MM.

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well... that is the problem... is it not? I was thinking about,

0.982/284+18x = N(no. of mole of whole sample)

also for Fe 55.8/284+18x = P/100 to find percentage and then find percentage gram from 0.982, unfortunately we have 2 variables...

Drake seemed as though he understood it, wish he would come on...

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This is the question which i found in one of the past papers that my teacher gave me, the question is clear :

The value of x in Fe(NH4 )2 (SO4 )2 . xH2 O can be found by determining the amount, in moles, of

sulfate in the compound.

A 0.982 g sample was dissolved in water and excess BaCl 2(aq) was added.

The precipitate of BaSO4 was separated and dried and found to weigh 1.17 g.

a)Calculate the amount, in moles of BaSO4 in the 1.17 g precipitate.

b)Calculate the amount, in moles, of sulfate in the 0.982 g sample of

Fe(NH4)2 (SO4 )2 . xH2O.

and then:

c)Calculate the amount, in moles, of iron in the 0.982 g sample of Fe(NH4 )2 (SO4 )2. xH2 O .

If you answer the first and the second the third (which you are asking about is direct)

if you answer the first by dividing 1.17/ M(BaSO4) you will get the n of BaSO4

then by using the chemical formula you will know that this number of moles will be the same as SO4 number of moles.

but you know that the number of moles of iron is half the number of mole of SO4 so by dividing your answer in b) by two you will get the answer.

This is a rough plan for answering the question, if you don&#39;t understand and want me to elaborate then i will.

Note: Please pay attention before writing the question to give us a full information about it. Because you are wasting our and your time. Thanks and we are always here to help.

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So what are the answers? 5.02 * 10 ^-3 for (a) and (b)?

Why is b the same answer?

And i dont get c. Can you please elaborate

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Thank you King! Only problem left is I can't download the link you put there...

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Then here is the full answer : or you can use the following link http://imgur.com/OzMCB

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Thanks but sorry

One question:

How do you know each 2 moles Sulphate corresponds to 1 mole Iron?

I thought the small subscripts dont relate to the number of moles.

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It relates them for inside the compound. If you had one mole of the compound with one iron in it then that's 1 mole of iron, 2 sulfates in the same compound would be double the number of particles of iron (because there's two of them) so that'd be 2 moles of sulfate. You could also kind of look at it as if it were to dissociate, you would make 1 mole of iron and 2 moles of sulfate once you're done balancing #### Share this post

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drake, what did you mean by water being a ligand?

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Where did I say this? O.o

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wait I also have a question to add on to this, how did you know that one mole of BaSO4 contains 1 mole of the SO4 for the compound> because the SO4 is given as 2 with subscript, wouldnt that mean that 2 SO4 in the compound is to 1 in the BaSO4? this is relating to part b hence i though we had to multiply the moles by 2

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We're learning mole at school right now. What my teacher told us and I found really helpful is CANCEL UNIT. It means when you do the calculation, you do not simply calculate the numbers, you also calculate the unit. For example: the unit of speed is m/s, and the unit of time is s, speed multiplies time equals distance. Why? It's beacuse when you multiply the two unit(m/s*s=s), the s is cancelled, and it becomes the unit of distance. Same in mole, the unit of mass is g, then when it's divided by g/mol(that's the molar mass), it becomes the unit of the number of mole. In this way, you won't be struggle with those boring numbers.

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