Determining the formula of a hydrated salt question Watch

nettogrof
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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.
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charco
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(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'.
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MexicanKeith
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(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!
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nettogrof
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Thank you charco and MexicanKeith
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