Question

Molecular Mass of ( mathrm{Na}_{2} mathrm{SO}_{4} ) is ( =2 mathrm{X} 23+32+4 mathrm{X} 16=142 mathrm{gm} )
To know the mass of ( mathrm{Na}_{2} mathrm{SO}_{4} ) containing ( 55.9 % mathrm{H}_{2} mathrm{O} ), we subtract 55.9 from 100 . ( 100-55.9=44.1 )
Therefore ( 44.1 mathrm{g} mathrm{Na}_{2} mathrm{SO}_{4} ) contains ( -55.9 mathrm{gm} mathrm{H}_{2} mathrm{O} )
So, ( 1 mathrm{gm} mathrm{Na}_{2} mathrm{SO}_{4} ) contains ( -mathrm{5} 5.9 / 44.1 mathrm{gm} mathrm{H}_{2} mathrm{O} )
142 gm Na2SO4 contains mes.9 ( / 44.1 times 142=180 mathrm{gm} mathrm{H}_{2} mathrm{O} ).
Now we have to find the number of water molecules. This will be the value of ( n ) as is asked in the question Number of water molecules ( =180 / 18 ) (since Molecular mass of ( mathrm{H}_{2} mathrm{O} ) is 18 ). ( =10 ) water molecules Hence the value of ( mathrm{n} ) is 10 and the formula is ( mathrm{Na}_{2} mathrm{SO}_{4} .10 mathrm{H}_{2} mathrm{O} ).

# On heating Na SO,.nH,0 it losses 55.9% of its weight then find the value of n (A) 5 (B)3 (C)7 (D) 10

Solution