State whether the statement is true or false. 18 g of water vapour and 18 g of ice will contain the same number of molecules.

Answer

To determine whether the statement "18 g of water vapour and 18 g of ice will contain the same number of molecules" is true or false, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Chemical Formula**:
The chemical formula for water is H₂O.

2. **Calculate the Molar Mass of Water**:
The molar mass of water (H₂O) can be calculated as follows:
- Hydrogen (H) has an atomic mass of approximately 1 g/mol. Since there are 2 hydrogen atoms, the total mass from hydrogen is:
\[ 2 \times 1 \text{ g/mol} = 2 \text{ g/mol} \]
- Oxygen (O) has an atomic mass of approximately 16 g/mol. Therefore, the total mass from oxygen is:
\[ 16 \text{ g/mol} \]
- Adding these together gives:
\[ \text{Molar mass of H₂O} = 2 \text{ g/mol} + 16 \text{ g/mol} = 18 \text{ g/mol} \]

3. **Determine the Number of Moles in 18 g of Water**:
Using the formula for calculating the number of moles:
\[ \text{Number of moles} = \frac{\text{Given mass}}{\text{Molar mass}} \]
For 18 g of water:
\[ \text{Number of moles} = \frac{18 \text{ g}}{18 \text{ g/mol}} = 1 \text{ mol} \]

4. **Calculate the Number of Molecules**:
According to Avogadro's number, 1 mole of any substance contains approximately \(6.02 \times 10^{23}\) molecules. Therefore, 1 mole of water (whether in the form of ice or water vapour) contains:
\[ 1 \text{ mol} \times 6.02 \times 10^{23} \text{ molecules/mol} = 6.02 \times 10^{23} \text{ molecules} \]

5. **Conclusion**:
Since both 18 g of water vapour and 18 g of ice are 1 mole of H₂O, they will contain the same number of molecules, which is \(6.02 \times 10^{23}\).

Thus, the statement "18 g of water vapour and 18 g of ice will contain the same number of molecules" is **TRUE**.