The molality of pure water is

Options

A. 55.5
B. 50.5
C. 18
D. 60.5

Answer

To find the molality of pure water, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Molality**:
- Molality (m) is defined as the number of moles of solute per kilogram of solvent.
- The formula is given by:
\[ m = \frac{\text{Number of moles of solute}}{\text{Weight of solvent in kg}} \]

2. **Identify the Solute and Solvent**:
- In this case, since we are dealing with pure water, both the solute and solvent are water.

3. **Assume a Quantity of Water**:
- Let’s assume we have \( x \) kg of pure water. For simplicity, we can take \( x = 1 \) kg.

4. **Calculate the Number of Moles of Water**:
- The number of moles (n) can be calculated using the formula:
\[ n = \frac{\text{Weight of water in grams}}{\text{Molecular weight of water in grams per mole}} \]
- Since we assumed \( x = 1 \) kg, we convert this to grams:
\[ \text{Weight of water} = 1 \text{ kg} = 1000 \text{ grams} \]
- The molecular weight of water (H₂O) is:
- Hydrogen: 1 g/mol × 2 = 2 g/mol
- Oxygen: 16 g/mol × 1 = 16 g/mol
- Total = 2 + 16 = 18 g/mol

5. **Substituting Values to Find Moles**:
- Now substituting the values into the moles formula:
\[ n = \frac{1000 \text{ grams}}{18 \text{ g/mol}} \approx 55.56 \text{ moles} \]

6. **Calculate the Molality**:
- Now we can substitute the number of moles and the weight of the solvent into the molality formula:
\[ m = \frac{55.56 \text{ moles}}{1 \text{ kg}} = 55.56 \]

7. **Final Result**:
- Therefore, the molality of pure water is approximately:
\[ m \approx 55.56 \]

### Conclusion:
The molality of pure water is **55.56**.