5g of a crystalline salt when rendered anhydrous lost 1.8G of water. formula weight of the anhydrous salt is 160. The number of molecules of water of crystallization in the salt is

Given,
      5g of a crystalline salt when rendered anhydrous lost 1.8g of water
Therefore,
      Amount of anhydrous salt left = 5 -1.8 = 3.2 g
      Molecular weight = 160 g/mol
Thus,
      Number of moles of anhydrous salt = (3.2g) / (160 g/mol)
                                                                =  0.02 moles
Mass of water lost = 1.8 g
Molar mass of water  = 18 g/mol
Thus,
      Number of moles of water = (1.8g) / (18 g/mol)
                                                 =  0.1 moles
Ratio of the anhydrous salt to water is ,
      Salt : Water
     = 0.02 moles : 0.1 moles

Divide by the smallest number,
     (0.02 moles / 0.02)  :  (0.1 moles / 0.1)
                                 1  :  5
Hence,
    
The number of molecules of water of crystallisation in the salt = 5 H2O
 

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