Which of the following best describes the difference between molarity and molality?

• A. Molarity is defined as moles of solute per liter of solution, while molality is defined as moles of solute per kilogram of solvent.
• B. Molarity and molality are both defined as moles of solute per liter of solvent.
• C. Molarity and molality are both defined as moles of solute per kilogram of solution.
• D. Molarity is defined as moles of solute per kilogram of solvent, while molality is defined as moles of solute per liter of solution.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the definitions of molarity and molality. Molarity (M) is the number of moles of solute divided by the volume of the entire solution in liters.
2. Step 2: Write the formula for molarity: \(M = \frac{\text{moles of solute}}{\text{liters of solution}}\).
3. Step 3: Understand that molality (m) is the number of moles of solute divided by the mass of the solvent in kilograms, not the solution.
4. Step 4: Write the formula for molality: \(m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}\).
5. Step 5: Compare the two definitions carefully: molarity depends on the volume of the solution, while molality depends on the mass of the solvent. This distinction is important because volume can change with temperature, but mass does not.