Question

which of the following has the highest percentage of water of crystallization c a s o4 into 2 H2O and a l 2 or 3 into 2 H2O

Answer 1

Assume 100.0 g of CaSO4 XH2O.  

20.9 % of that = 20.9 g  

That is the mass of water.  

1 mole of water = 18.0 g  

20.9 g x 1 mol/18.0 g = 1.16 mol H2O  

100 % - 20.9 % = 79.1 % CaSO4  

79.1 g is the mass of CaSO4  

1 mole of CaSO4 = 40.1 + 32.1 + 4(16.0) = 136.2 g  

79.1 g x 1 mol / 136.2 g = 0.581 mol CaSO4  

Now calculate the ratio of mol H2O to mol CaSO4  

1.16 / 0.581 = 2.00  

So X = 2

hope its clear