Question

why mass of solvent is used in molality rather than mass of solution.​

Answer 1

Molality is expressed in moles of solute per kilogram of solvent, while molarity is expressed as moles of solute per liter of solution.

Molarity is dependent on temperature, since the quantity of the solution is based on volume, and volume is a function of temperature.

The mass of a substance however is independent of temperature; this is what makes molalityuseful in a variety of situations that involve a drastic pressure or temperature change, which can change the solution's volume.

In such situations, the molarity would change, but the molality would remain unchanged. Thus, molality is often used in comparing and determining a solution's colligative properties.

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