A sample of hydrated copper sulphate is heated to drive off the water of crystallization, cooled and reweighed 0.869 g of `CuSO_(4)`a`H_(2)`O gave a residue of 0.556 g. Find the molecular formula of hydrated copper sulphate.

Explanation

0.869 g of `CuSO_(4)*aH_(2)O` gave a residue of 0.556 g of Anhydrous `CuSO_(4)` `:` Weight of a `H_(2)O` molecule = 0.869-0.556 = 0.313 g
Molecular weight of `H_(2)O = (1xx2)+16=2+16=18`
No. of moles of water =`("Mass")/("Molecular mass")`
`CuSO_(4)*5H_(2)O` - Molecular mass = 63.5 + 32 + 64 + 90 = 249.5 g
249.5 g of `CuSO_(4) .5H_(2)O` on heating gives 159.5 g of `CuSO_(4)`
. 0.869 g of `CuSO_(4)*aH_(2)O` on heating gives
`159.5/249.5 xx 0.869`
= 0.556 g of anhydrous `CuSO_(4)`
`:.` a =5
The molecular formula of hydrated copper sulphate = `CuSO_(4)*5H_(2)O.`