The formula of soda ash is

1.  Na2CO3·10H2O

2.  Na2CO3·2H2O

3.  Na2CO3·H2O

4.  Na2CO3


 
 

Hint: Soda ash is anhydrous sodium carbonate

On heating washing soda, it loses its water of crystallisation. Above 373 K, it becomes a completely anhydrous white powder called soda ash.
Na2CO3·10H2OWashing soda>373 KNa2CO3Soda ash+ 10H2O