What is molality a measure of?

Molality is defined as the number of moles of solute per kilogram of solvent. This measurement focuses specifically on how concentrated a solution is in terms of the solute present in a given mass of solvent. In this context, it provides a way to express concentration that remains unaffected by temperature changes, which is particularly useful in situations where temperature variations might alter the volume of the solution.

In this case, when considering the formula for molality, it can be expressed as:

[ \text{Molality (m)} = \frac{\text{Moles of solute}}{\text{Mass of solvent in kg}} ]

This means that molality directly relates the amount of solute to a fixed mass of solvent, making it advantageous for calculations in colligative properties and for reactions occurring in solutions.

The other options do not accurately define molality: it does not concern the volume of solvent, nor does it measure mass of solute per liter of solution, which would instead relate to molarity. Additionally, density pertains to mass per unit volume, which is not relevant to the definition of molality. Understanding these distinctions clarifies why the option regarding moles of solute per kilogram of solvent is the correct choice.