The decahydrate form of sodium carbonate i.e. washing soda on standing in air effloresces and crumbles to powder. The number of water molecule (s) present in the compound formed is :

Answer

To solve the problem, we need to understand the process of efflorescence of sodium carbonate decahydrate (washing soda) when it is exposed to air. Here’s a step-by-step solution:

### Step-by-Step Solution:

1. **Identify the Compound**: The compound in question is sodium carbonate decahydrate, which is represented as Na2CO3·10H2O. This indicates that each formula unit of sodium carbonate is associated with 10 water molecules.

2. **Understand Efflorescence**: Efflorescence is the process where a hydrate loses water molecules to the atmosphere when exposed to air. In the case of sodium carbonate decahydrate, it loses water molecules upon standing in air.

3. **Determine the Number of Water Molecules Lost**: According to the information provided, when sodium carbonate decahydrate is left in the air, it loses 9 water molecules. This means that it goes from Na2CO3·10H2O to Na2CO3·1H2O.

4. **Identify the Remaining Water Molecules**: After losing 9 water molecules, the remaining number of water molecules in the compound is 1.

5. **Conclusion**: Therefore, the number of water molecules present in the compound formed after efflorescence is 1.

### Final Answer:
The number of water molecules present in the compound formed is **1**.

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