What is the difference between molarity and molality?

What is the difference between molarity and molality?

• A. Molarity is moles per liter; Molality is moles per liter of solvent; one depends on volume, the other on volume.
• B. Molarity is moles per liter; Molality is moles per kilogram of solvent; one depends on volume, the other on mass.
• C. Molarity is moles per kilogram; Molality is moles per liter;
• D. They are identical.

Answer: (B)

The key idea is how concentration is defined and what quantity it’s tied to. Molarity uses liters of the entire solution, so it can change with temperature as the solution expands or contracts. Molality uses kilograms of solvent, which stays essentially constant with temperature, so it doesn’t depend on volume changes. Therefore, the correct distinction is that molarity is moles per liter of solution, and molality is moles per kilogram of solvent; one depends on volume, the other on mass.

The key idea is how concentration is defined and what quantity it’s tied to. Molarity uses liters of the entire solution, so it can change with temperature as the solution expands or contracts. Molality uses kilograms of solvent, which stays essentially constant with temperature, so it doesn’t depend on volume changes. Therefore, the correct distinction is that molarity is moles per liter of solution, and molality is moles per kilogram of solvent; one depends on volume, the other on mass.