The quantity remaining constant on dilution is

Options

A. Number of moles of solute
B. Molarity of the solution
C. Mole fraction of solute
D. Mass of the solution

Answer

To solve the question "The quantity remaining constant on dilution is?", we need to analyze the options provided and understand the concept of dilution.

### Step-by-Step Solution:

1. **Understanding Dilution**:
- Dilution is the process of adding more solvent to a solution, which increases the volume of the solution and decreases its concentration.

2. **Analyzing the Options**:
- **Option 1: Number of moles of solute**:
- The number of moles of solute is calculated using the formula:
\[ \text{Number of moles} = \frac{\text{mass of solute}}{\text{molar mass of solute}} \]
- During dilution, we only add solvent; the amount of solute remains unchanged. Therefore, the number of moles of solute stays constant.

- **Option 2: Molarity of the solution**:
- Molarity (M) is defined as:
\[ M = \frac{\text{Number of moles of solute}}{\text{Volume of solution in liters}} \]
- Since we are increasing the volume of the solution by adding solvent, the molarity will decrease. Thus, molarity does not remain constant.

- **Option 3: Mole fraction of solute**:
- The mole fraction of solute (X_solute) is given by:
\[ X_{\text{solute}} = \frac{\text{Number of moles of solute}}{\text{Total number of moles}} \]
- As we add solvent, the total number of moles (which includes moles of solvent) changes, thus the mole fraction of solute will also change. Therefore, it does not remain constant.

- **Option 4: Mass of the solution**:
- The mass of the solution is the sum of the mass of the solute and the mass of the solvent. When we add more solvent, the mass of the solution increases. Hence, the mass of the solution does not remain constant.

3. **Conclusion**:
- The only quantity that remains constant during dilution is the **number of moles of solute**. Therefore, the correct answer is **Option 1: Number of moles of solute**.

### Final Answer:
The quantity remaining constant on dilution is the **number of moles of solute**.