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# Determining the formula of a hydrated salt question

1. 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.
2. (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'.
3. (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!
4. Thank you charco and MexicanKeith

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