Answer :
The percent by mass of H₂O in the hydrated compound, Na₂CO₃.xH₂O = 64.13%
Further explanation
Given
90 g sample Na₂CO₃.xH₂O
32.28 g Na₂CO₃
Required
The percent by mass of H₂O
Solution
mass of H₂O = mass of sample - mass of the anhydrous salt, Na₂CO₃
[tex]\tt mass~H_2O=90-32.28=57.72~g[/tex]
The percent by mass :
[tex]\tt \%mass~H_2O=\dfrac{mass~H_2O}{mass~sample}\times 100\%\\\\\%mass~H_2O=\dfrac{57.72}{90}\times 100\%\\\\\%mass~H_2O=64.13\%[/tex]