1.615g of anhydrous ZnSO4 was placed in moist air. After a few days, its mass was found to be 2.875g. What is the molecular formula of the hydrated salt?
Solution:
The quantity of anhydrous ZnSO4, that is, devoid of any water molecule, is 1.615 g. 
The difference in mass due to water is,
H2O = 2.875 g – 1.615 g = 1.26 g. 
Molecular mass of anhydrous ZnSO4 is = 65.5 + 32 + 64 = 161.5

The molecular formula of the hydrated salt is ZnSO4.7H2O. 
It means that, one molecule of ZnSO4 traps seven molecules of water to form the hydrated salt.
Second method
1.615 g of anhydrous ZnSO4 combine with 1.216 g of water.
Therefore, 161.5 g of anhydrous ZnSO4 will combine with ? water
= (161.5 x 1.216)/1.615 = 126 g of water
In moles, 126g /18 (MW of water) = 7 moles.
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