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Determining the formula of a hydrated salt question

A hydrated salt of aluminium sulfate, Al2(SO4)3 . xH2O contains 48.63% of water by mass. Find its formula.

Please could someone show me how to work this out.
Original post by nettogrof
A hydrated salt of aluminium sulfate, Al2(SO4)3 . xH2O contains 48.63% of water by mass. Find its formula.

Please could someone show me how to work this out.


Imagine that there are 100g of the hydrated salt.
The Al2(SO4)3 will have a mass of 100 - 48.63 g
and the water has a mass of 48.63 g

You can convert each to a number of moles by dividing by the relative formula mass.

You then have a ratio of moles of each.

You divide both values by the moles of the aluminium sulfate (to make this equal to 1) and the moles of water now equals 'x'.
Original post by nettogrof
A hydrated salt of aluminium sulfate, Al2(SO4)3 . xH2O contains 48.63% of water by mass. Find its formula.

Please could someone show me how to work this out.


I would start by calculating the formula mass of the anhydrous salt (ie the mass of Al2(SO4)3)
You know this mass must be the other 51.37% of the hydrated salt so you can work out the mass of water in one formula unit.

Hopefully that helps, if you need a few more pointers just ask! :smile:
(edited 7 years ago)
Reply 3
Avatar for nettogrof
nettogrof
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Thank you @charco and @MexicanKeith
Mass percentages of compounds:48.63% = H2O So 100%-48.63% = 51.37%So51.37% = Al2(SO4)3 2) Find the Mr (Molecular relative mass)H2O = (1x2) 16 = 18g/mol Al2(SO4)3 = (27x2) 3(32 16x4) = 342g/mol3) Find number of moles for each compound:h:2O = 48.63/18 = 2.7molAl2(SO4)3 = 0.15mol4) Find the molar ratio by dividing each number of moles by the smallest number of moles.Smallest number of moles = 0.15molSoH2O = 2.7/0.15 = 18Al2(SO4)3 = 1So the molar ratio isAl2(SO4)3 : H2O 1 : 8So the formula for the hydrated salt is: Al2(SO4)3 • 8H2O
Original post by StudentLearns
Mass percentages of compounds:48.63% = H2O So 100%-48.63% = 51.37%So51.37% = Al2(SO4)3 2) Find the Mr (Molecular relative mass)H2O = (1x2) 16 = 18g/mol Al2(SO4)3 = (27x2) 3(32 16x4) = 342g/mol3) Find number of moles for each compound:h:2O = 48.63/18 = 2.7molAl2(SO4)3 = 0.15mol4) Find the molar ratio by dividing each number of moles by the smallest number of moles.Smallest number of moles = 0.15molSoH2O = 2.7/0.15 = 18Al2(SO4)3 = 1So the molar ratio isAl2(SO4)3 : H2O 1 : 8So the formula for the hydrated salt is: Al2(SO4)3 • 8H2O

That’s how you do the question (my answer)^^
I am studying A level Chemistry. Does this come up in A level too?
Original post by StudentLearns
Mass percentages of compounds:48.63% = H2O So 100%-48.63% = 51.37%So51.37% = Al2(SO4)3 2) Find the Mr (Molecular relative mass)H2O = (1x2) 16 = 18g/mol Al2(SO4)3 = (27x2) 3(32 16x4) = 342g/mol3) Find number of moles for each compound:h:2O = 48.63/18 = 2.7molAl2(SO4)3 = 0.15mol4) Find the molar ratio by dividing each number of moles by the smallest number of moles.Smallest number of moles = 0.15molSoH2O = 2.7/0.15 = 18Al2(SO4)3 = 1So the molar ratio isAl2(SO4)3 : H2O 1 : 8So the formula for the hydrated salt is: Al2(SO4)3 • 8H2O

You got it wrong, buddy
How do u work it out then if it’s wrong
Original post by charco
You got it wrong, buddy

It is right. Go check the rest of the students comments, they followed the same step as me. If it is wrong, you can work it out and show us
Original post by StudentLearns
It is right. Go check the rest of the students comments, they followed the same step as me. If it is wrong, you can work it out and show us

I think you'll find that if you check the second post that I made over three years ago, I already have showed you how to work it out.

There is a more rapid way, though.

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