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Calculating the value of x in M. H2O

Hi can someone please explain this question to me for the empirical formula i got NH28 AlS2 O20
in the mark scheme the value of x was 12 . Iā€™m not sure how they got x to be 12
The question is belowB6F05980-B658-4791-89D4-CCB17C900BE1.jpeg
(edited 9 months ago)
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Reply 2
The empirical formula (as you mentioned) works out to be N H28 Al S2 O20. It then mentions the relative formula mass. When you add up the mass from the empirical formula, it equals to the given mass, meaning the empirical formula is the molecular formula.

It then mentions that the ions NH4+, Al3+, and SO42- are present. When you subtract these from the molecular formula (there are two SO42- ions so subtract 2 S and 8 O, there is 1 NH4+ ion so subtract 1 N and 4 H, and there is 1 Al 3+ so subtract 1 Al)

You are then left with
H 24 and O 12

you can then create 12 H2O molecules with this, hence the answer for x is 12. (24/2 H, 12O)

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