Question
Tue June 05, 2012 By: Pramod R
 

1.615g of an anhydrous salt was placed in moist air.After few days it was found to be 2.875g.Assuring that complete hydration has taken place,calculate the simplest formula of hydrated salt.The composition of anhydrous salt is Zn = 40.6%, S =19.8%, O = 39.6%.Please explain

Expert Reply
Sun June 24, 2012
Element  percentage  atomic mass   relative no of moles  simplest mole ratio whole number ratio
   Zn           40.6             65.5             40.6/65.5=0.619       0.619/0.618        1          
   S              19.8             32                0.618                       0.618 /0.618      1    
    O             39.6             16                2.475                        2.475/0.618        4
Formula = ZnSO4          
Molar mass of ZnSO4 = 161.5
now, 1.615g anhydrous salt combine withH2O  = 2.875-1.615 =1.260g
so,161.5g combine with H2O= 126g
no of moles oh H2O = 126/18 = 7 moles
Hence formula = ZnSO4.7H2O
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