11) D

Sodium carbonate = Na₂CO₃

Decahydrate means 10 moles of water. (deca = 10)

So the formula of sodium carbonate decahydrate = Na₂CO₃.10 H₂O

Mr of sodium carbonate decahydrate = (23 x 2) + (12 x 1) + (16 x 3) + (2 x 10) + (16 x 10)
(23 x 2) + (12 x 1) + (16 x 3) + (2 x 10) + (16 x 10) = 46 + 12 + 48 + 180

Since there are 10 moles of water, mass of water = (2 x 10) + (16 x 10) = 180

So the equation for the percentage by mass of water of crystalline in sodium carbonate decahydrate:

       180 x 100         
46 + 12 + 48 + 180

So the answer is D.