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

Explanation

To solve the problem, we need to understand the process of efflorescence and how it affects the hydrated form of sodium carbonate, which is also known as washing soda. Here’s a step-by-step solution:

### Step 1: Identify the Formula of Washing Soda
Washing soda is the decahydrate form of sodium carbonate, which has the chemical formula:
\[ \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \]

### Step 2: Understand Efflorescence
Efflorescence is the process where a hydrated compound loses water of crystallization when exposed to air. In this case, washing soda loses water molecules.

### Step 3: Determine the Loss of Water Molecules
When washing soda (Na2CO3·10H2O) is left in the air, it loses water molecules. According to the problem, it is stated that it crumbles to powder, indicating that it has lost a significant amount of its water content.

### Step 4: Calculate the Remaining Water Molecules
It is mentioned that 9 water molecules are lost during the efflorescence. Thus, we can calculate the remaining water molecules as follows:
\[ \text{Remaining water molecules} = 10 - 9 = 1 \]

### Step 5: Write the Formula of the Resulting Compound
After losing 9 water molecules, the remaining compound can be expressed as:
\[ \text{Na}_2\text{CO}_3 \cdot 1\text{H}_2\text{O} \]
This indicates that the compound formed is sodium carbonate monohydrate.

### Step 6: Conclusion
The number of water molecules present in the compound formed after efflorescence is:
\[ \text{1 water molecule} \]

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

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