The formula of soda ash is:
(A) $Na_{ 2 }CO_{ 3 }.10H_{ 2 }O$
(B) $Na_{ 2 }CO_{ 3 }.2H_{ 2 }O$
(C) $Na_{ 2 }CO_{ 3 }.H_{ 2 }O$
(D) $Na_{ 2 }CO_{ 3 }$

Hint: Both washing soda and soda ash are derived from sodium carbonate and they both have different numbers of water molecules as the water of crystallization. Name soda ash suggests that it is prepared from washing soda by heating.

Complete step by step answer:
> We know that sodium carbonate is reacted with water to form crystalline sodium carbonate which is nothing but washing soda. Soda ash name suggests that it is prepared from soda by heating. On strong heating washing, soda water of crystallization will be evaporated. Anhydrous sodium carbonate is called soda ash. Soda ash has no water of crystallization. Therefore, option D is correct.
> Preparation of soda ash from washing soda
\[Na_{ 2 }CO_{ 3 }.10H_{ 2 }O\quad +\quad heat\quad \rightarrow \quad Na_{ 2 }CO_{ 3 }\quad +\quad 10H_{ 2 }O\]

Additional information: Hydrated sodium carbonate is used as a detergent, it is used in the manufacture of soaps and paper. Hydrated sodium carbonate with a different number of water molecules of crystallization is used for different purposes. Sodium carbonate is a basic salt. Soda ash is used in the manufacturing of different types of glasses for houses, commercial buildings, and industrial purposes. Soda ash is used in purifying the air. It is used in the removal of harmful gases from the air. Soda ash is used as a sweetener in many chemical industries.

Note: Sodium carbonate crystals are subjected to a strong heat in preparation of soda ash so that no water molecules of crystallization remain. Sodium carbonate is used as a slag in blast furnaces for removing impurities.