Final answer:
The correct concentration unit to use in freezing point depression calculations is molality (m), because it does not vary with temperature and is therefore more accurate for these types of precise measurements.
Step-by-step explanation:
When determining the correct expression for the concentration units used in freezing point depression calculations, molality (m) is the appropriate unit to use. This unit measures the moles of solute per kilogram of solvent and is independent of temperature changes. Unlike molarity (M), which is temperature-dependent since it involves the volume of the solution, molality remains constant because it's based on mass, a non-variable property. For accurate and precise work, especially in colligative properties like freezing point depression, molality is the preferred unit.
The freezing point depression constant, a value specific to the solvent, relates the molality of a solution to the change in its freezing point. To calculate the change in freezing point, multiply the molality of the solution by the freezing point depression constant. This approach highlights the importance of using molality in such calculations to ensure accuracy without the need for temperature corrections.