A crystalline salt when heated becomes anhydrous and losses 51.2% of its weight. The anhydrous salt on analysis gave the percentage composition as Mg= 20.0%, S= 26.66% and 0=53.33%.
The empirical formula of the anhydrous salt comes out to be MgSO_{4}. Empirical formula mass = 120.
Molecular mass = 120.
Hence, molecular formula = MgSO_{4}.
As crystalline salt on becoming anhydrous loses 51.2% by mass, this means 48.8 g of anhydrous salt contains H _{ 2 }O = 51.2 g. Therefore, 120 g of anhydrous salt contains
51.2 / 48.8 x 120 g = 126 g = 126/18 molecules = 7 H _{ 2 }O molecules
Hence, molecular formula of crystalline salt = MgSO_{4} • 7$H _{ 2 }O$
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