Question

The decahydrate form of sodium carbonate i.e. washing soda on standing in air efflorescence and crumbled to powder. The number of water molecules present in the compound formed is:

Answer 1

. Sodium carbonate is an inorganic compound having formula $N{{a}_{2}}C{{O}_{3}}$ which is also known by names washing soda, soda ash and soda crystals. These all forms are white water soluble salt that yield moderately alkaline solution in water.

Complete step by step answer:
Generally sodium carbonate is obtained as three hydrates which are as:
Sodium carbonate decahydrate which is shown by molecular formula $N{{a}_{2}}C{{O}_{3}}.10{{H}_{2}}O$ it readily efflorescence to form the monohydrate.
Sodium carbonate heptahydrate shown by molecular formula $N{{a}_{2}}C{{O}_{3}}.7{{H}_{2}}O$.
Sodium carbonate monohydrate known as thermonatrite represented by $N{{a}_{2}}C{{O}_{3}}.{{H}_{2}}O$ also known as crystal carbonate.

The decahydrate form of sodium carbonate i.e. washing soda represented by $N{{a}_{2}}C{{O}_{3}}.10{{H}_{2}}O$ is transparent crystalline solid which is soluble in water. It is alkaline in nature with a pH level of 11 i.e. basic in nature which turns red litmus to blue. It has detergent properties or cleansing properties, because it can remove dirt and grease from dirty clothes, etc. It attacks dirt and grease to form water soluble products which are then washed away on rinsing with water.
Washing soda has 10 water molecules as water of crystallization. Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. When it is exposed to air, it loses its 9 molecules of crystallization and becomes sodium carbonate monohydrate. Reaction can be shown as:
$N{{a}_{2}}C{{O}_{3}}.10{{H}_{2}}O\to N{{a}_{2}}C{{O}_{3}}.{{H}_{2}}O+9{{H}_{2}}O$

Hence the number of water molecules present in the compound formed is one known by sodium carbonate monohydrate.

Note: Other than washing soda sodium carbonate can also be used for removing temporary and permanent hardness of water, used in the manufacture of glass, soap and paper and also used in the manufacture of sodium compounds like borax.