Which does not change on dilution?

Options

A. Molarity oof solution
B. Molality of solution
C. Milli-moles and milli-equivalent of solution
D. Mole fraction of solute

Explanation

To determine which quantity does not change upon dilution, we need to analyze each of the given options carefully.

### Step-by-Step Solution:

1. **Understanding Dilution**:
- Dilution involves adding more solvent to a solution, which increases the volume of the solution while keeping the amount of solute constant.

2. **Option 1: Molarity (M)**:
- Molarity is defined as the number of moles of solute per liter of solution (M = moles of solute / volume of solution in liters).
- When we dilute a solution, the volume of the solution increases, which causes the molarity to decrease.
- **Conclusion**: Molarity changes upon dilution. (This option is incorrect.)

3. **Option 2: Molality (m)**:
- Molality is defined as the number of moles of solute per kilogram of solvent (m = moles of solute / mass of solvent in kg).
- When we dilute a solution, we are adding more solvent, which increases the mass of the solvent.
- **Conclusion**: Molality changes upon dilution. (This option is incorrect.)

4. **Option 3: Millimoles and Milliequivalents**:
- Millimoles are a measure of the amount of substance (1 millimole = 0.001 moles), and milliequivalents are a measure related to the reactive capacity of the solute.
- When we dilute a solution, the total number of millimoles and milliequivalents of solute remains constant because we are not changing the amount of solute, only the volume of the solution.
- **Conclusion**: Millimoles and milliequivalents do not change upon dilution. (This option is correct.)

5. **Option 4: Mole Fraction of Solute**:
- Mole fraction is defined as the ratio of the number of moles of solute to the total number of moles of all components in the solution.
- When we dilute a solution, the number of moles of solute remains the same, but the total number of moles (solute + solvent) increases, which affects the mole fraction.
- **Conclusion**: Mole fraction changes upon dilution. (This option is incorrect.)

### Final Answer:
The quantity that does not change upon dilution is **Millimoles and Milliequivalents**.

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