UPSEE Solved Model Paper 7

Given
Hydrated salt Na2SO4.nH2O
Loss of weight on heating = 55.9 %
We have to find the value of 'n'.
Now, gram molecular mass of anhydrous salt, Na2SO4 = 2 × 23 + 32 + 4 × 16
= 46 + 32 + 64
= 142 g
Let mass of hydrated salt be x g.
The loss of weight on heating is due to loss of water molecules, thus.
Mass of hydrated salt -

55.9

100

× mass of hydrated salt = Mass of anhydrous salt
x -

55.9x

100

= 142

44.1x

100

= 142
x = 322 g
Thus, mass of H2O in Na2SO4.nH2O = Mass of Na2SO4.nH2O - Gram molecular Mass of Na2SO4
= 322 - 142
= 180 g
Now, molecular mass of H2O = 2 × 1 + 16
= 18 g
Now, number of moles is given as :
Number of moles =

Given‌mass

Molecular‌mass

Thus, moles of water , n =

180

18

= 10
Therefore, the formula of Na2SO4.nH2O is Na2SO4.10H2O
Hence, option 'D' is correct.