Which of the following methods of expressing concentration varies with temperature ?

Options

A. Molality
B. Weight percent
C. Molarity
D. Mole fraction

Answer

To determine which method of expressing concentration varies with temperature, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Molality (Option A)**:
- Molality is defined as the number of moles of solute per kilogram of solvent.
- Formula: \( \text{Molality} = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}} \)
- Since the number of moles of solute and the mass of the solvent do not change with temperature, molality does not vary with temperature.
- **Conclusion**: Option A is incorrect.

2. **Understanding Weight Percent (Option B)**:
- Weight percent is defined as the mass of solute divided by the total mass of the solution, multiplied by 100.
- Formula: \( \text{Weight Percent} = \left( \frac{\text{weight of solute}}{\text{weight of solution}} \right) \times 100 \)
- Both the weight of the solute and the weight of the solution are not affected by temperature. Thus, weight percent remains constant with temperature.
- **Conclusion**: Option B is incorrect.

3. **Understanding Molarity (Option C)**:
- Molarity is defined as the number of moles of solute per liter of solution.
- Formula: \( \text{Molarity} = \frac{\text{moles of solute}}{\text{volume of solution (L)}} \)
- Volume of the solution is temperature-dependent; as temperature increases, the volume typically increases. Therefore, if the volume increases while the number of moles remains constant, the molarity will decrease.
- **Conclusion**: Option C is correct as molarity varies with temperature.

4. **Understanding Mole Fraction (Option D)**:
- Mole fraction is defined as the ratio of the number of moles of a component to the total number of moles in the solution.
- Formula: \( \text{Mole Fraction} = \frac{\text{moles of component}}{\text{total moles of solution}} \)
- The number of moles does not depend on temperature, so mole fraction remains unchanged with temperature.
- **Conclusion**: Option D is incorrect.

### Final Answer:
The method of expressing concentration that varies with temperature is **Option C: Molarity**.