Question

26.8 g Na2SO4.nH2O has 12.6 g water.find the value of n

Answer 1

To get the value of n; calculate the moles of water and that of sodium sulphate and then get the whole number ratio:
Moles of water:
mass H2O=12.6
molar mass= 18    hence moles
moles=mass/molar mass
          =12.6/18
         =0.7
Moles of sodium sulphate:

mass= 26.8-12.6=14.2
molar mass= (23x2)+(32+64)=142
 
there its moles =14.2/142
                          =0.1
there ration of moles of Na2SO4:H2O =0.1:0.7
hence the whole number ration =    1:7 (meaning there are 7 molecules of water in this hydrated salt)

hence n=7