The formula of soda ash is

1.  ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}·10{\mathrm{H}}_{2}\mathrm{O}$

2.  ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}·2{\mathrm{H}}_{2}\mathrm{O}$

3.  ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}·{\mathrm{H}}_{2}\mathrm{O}$

4.  ${\mathrm{Na}}_{2}{\mathrm{CO}}_{3}$

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.