The percent by mass of water in Na2So4 10H2O is 55.9%
calculation
% mass = molar mass of water(H2O) in Na2SO4 .10H2O/ molar mass of Na2SO4. 10H2O x100
that is mass of H2O = 10{ ( 1x2) + 16} = 180 g/ mol
mass of Na2SO4.H2O= { (2 x23)+ 32 + ( 16 x4 ) + 180 = 322 g/mol
therefore the % mass composition = 180/322 x100 =55.9 %
