Question

The decahydrate form of sodium carbonate i.e. washing soda on standing in air effloresces and crumbles to powder. The number of water molecule(s) present in the compound formed is:

Answer 1

Answer: 1

Hydrated crystals of sodium carbonate exposed to air it losses it is 9 molecules of water of crystallization leaving only powder sodium carbonate with one molecule of water
The process of losing water of crystallization is called efflorescence
$N{a}_{2}C{O}_3\cdot 10H_{2}O(s)($ decahydrate $)\stackrel{\text{ effloresces }-9H_{2}O}{\to }$
$N{a}_{2}C{O}_3\cdot H_{2}O(s)$ (Monohydrate)