The chemical formula for washing soda is ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$, which means that each molecule of washing soda contains $10$ water molecules.

The chemical formula for soda ash is ${\mathrm{Na}}_2{\mathrm{CO}}_3$, which does not contain any water molecules.

Therefore, there are $10$ water molecules in washing soda and $0$water molecules in soda ash.