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does anyone do edexcel chemistry?
the crystallisation practical?

Hey guys, i'm really stuck on this question.

MgSO4*7H2O has a mass of 4.2g

MgSO4 alone is 2g

Work out the percentage by mass of water? Does anyone know how to do this? Could list some steps into how I can get the answer?

Thanks

Reply 1882

C0balt

Original post by JackP32

Isn't it just subtracting 2 from 4.2 to get mass of water and dividing it by 4.2 then times 100

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Reply 1883

JackP32

Is it as easy as that? I thought I had to use molar masses of water? :confused:

Reply 1884

C0balt

Original post by JackP32

Is it as easy as that? I thought I had to use molar masses of water? :confused:

I don't know but where else could that extra mass come from

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Reply 1885

Dylann

Original post by JackP32

Is it as easy as that? I thought I had to use molar masses of water? :confused:

Original post by C0balt

Isn't it just subtracting 2 from 4.2 to get mass of water and dividing it by 4.2 then times 100

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It's more accurate to use the Mr

Mr of the waters = (7x18)
Mr of the whole hydrated compound = 24.3+32.1+64+(7x18)=246.4

(7x18)/246.4 = 0.511... x100 = 51.1%

Notice how this is different to the experimental value of 2.2/4.2 x 100 = 52.3%

Reply 1886

JackP32

I have my coursework tomorrow, which method do you suggest I use? or does it matter?

Reply 1887

C0balt

Original post by Dylann

I just noticed that
I also did working amount of mole of 2g MgSO4 out and multiplying with 7 then dividing which gives 50.0% lol it's interesting

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Reply 1888

Dylann

Original post by JackP32

MgSO4 alone is 2g

Work out the percentage by mass of water? Does anyone know how to do this? Could list some steps into how I can get the answer?

Thanks

Original post by C0balt

I just noticed that
I also did working amount of mole of 2g MgSO4 out and multiplying with 7 then dividing which gives 50.0% lol it's interesting

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It's probably due to the assumption that every hydrated MgSO4 contains 7 water molecules...when actually it's likely some are 7 and some are 8.

If you like a bit of simple maths, you can work out what the average number of water molecules are given the experimental data...I've put the calculations in a spoiler if you wanna have a go before you look at the answer

Spoiler

Reply 1889

C0balt

Original post by Dylann

If the mass of MgSO4 is 2 grams, then the moles of MgSO4 is

mol = mass/mr (note how I got the correct equation this time lol)

mol = 2/120.4
mol = 5/301 moles of MgSO4

If the mass of H2O is 2.2 grams, then the moles of H2O is

mol = 2.2/18
mol = 0.12222....

since 5/301 is smaller than 2.2/18, we can divide them both by 5/301 to find a molar ratio

2.2/18 divided by 5/301 is 7.35777....

So for every 1 mole of MgSO4, there are 7.3577 moles of H2O. That means that most of compounds contain 7 waters, but a considerable number contain 8.

We can prove this further by repeating the Mr calculation, using our new number of waters:

(7.3577 x 18)/120.4 + (7.3577 x 18) = 0.05238....x100 = 52.38% which was the same as the experimental calculation (2.2/4.2 x 100)
:biggrin:

Yup, looks like you are awake this time :wink:

lol

I didn't know it could contain 8, I just assumed it to be some experimental error or impurity so thanks for that :lol:

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Reply 1890

Dylann

Original post by C0balt

Yup, looks like you are awake this time :wink:

lol

I didn't know it could contain 8, I just assumed it to be some experimental error or impurity so thanks for that :lol:

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And yeah, impurity or weighing error is probably quite likely too. BTW which exam board are you on?

Reply 1891

C0balt

Original post by Dylann

And yeah, impurity or weighing error is probably quite likely too. BTW which exam board are you on?

Edexcel

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Reply 1892

samb1234

Original post by Dylann

This is not really technically more accurate. The question gives you experimental data for a reason, and if in the exam you decided to ignore that data and use the Mr you could easily lose marks

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Reply 1893

Dylann

Original post by samb1234

I was referring to the Mr calculation being more accurate, since it assumes a pure sample. Obviously the experimental data would be the one to use in the exam.

Reply 1894

samb1234

Original post by Dylann

I was referring to the Mr calculation being more accurate, since it assumes a pure sample. Obviously the experimental data would be the one to use in the exam.

I understand what you're saying its just an over complication especially if you didn't know how to do it in the first place

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Reply 1895

langlitz

Original post by samb1234

I understand what you're saying its just an over complication especially if you didn't know how to do it in the first placePosted from TSR Mobile

Dylann just thinks he's the dogs *******s with his A2 chemistry, wants to show off his skills haha

Reply 1896

ilovecake123

please can someone help me with his question:
when 0.1 mol of atoms of an element reacts with chlorine there is an increase in mass of 7.1g.
the element could be
a.carbon
b.sodium
c.magnesium
d.aluminium

thanks any help would be appreciated!!

Reply 1897

C0balt

Original post by ilovecake123

Find the number of moles of chlorine in the compound.
Now you can deduce the ratio of the element to chlorine
Which of those element will form a compound with chlorine in that ratio?

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Reply 1898

ilovecake123

Original post by C0balt

how can i find the number of chlorine molecules of the compound? i don't know the mass of it?
and if i did do you divide by 71/35.5?

(edited 9 years ago)

Reply 1899

C0balt

Original post by ilovecake123

how can i find the number of chlorine molecules of the compound? i don't know the mass of it?

The element had an increase in mass by 7.1g when reacted with chlorine. The only source of the increase is chlorine that has reacted with the element. You know the relative atomic mass of Chlorine from the periodic table.

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