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Determining the formula of a hydrated salt question

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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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    (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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    (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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    Thank you charco and MexicanKeith
 
 
 
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