Sodium carbonate exists in different forms, each with a different number of water molecules of crystallization. Here's a breakdown:

* Sodium carbonate anhydrous (Na₂CO₃): No water molecules are present.

* Sodium carbonate monohydrate (Na₂CO₃·H₂O): One water molecule per unit of sodium carbonate.

* Sodium carbonate decahydrate (Na₂CO₃·10H₂O): Ten water molecules per unit of sodium carbonate.

Therefore, the number of water molecules of crystallization per unit of sodium carbonate depends on the specific form.

Let me know if you'd like to know more about any of these forms!