The correct option is Moles of solute per kilogram of solvent.
Explanation
Concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Molality (m) is a preferred unit of concentration in experiments involving temperature changes because it relies on mass rather than volume.
Option Analysis
- Moles of solute per kilogram of solvent is Correct: Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent. The formula is:
Molality (m) = Moles of Solute / Mass of Solvent (in kg). - Moles of solute per litre of solvent is Incorrect: Moles of solute per litre of solvent is not a standard concentration term.
- Moles of solute per kilogram of solution is Incorrect: Moles of solute per kilogram of solution is not the definition of molality; molality specifically considers the mass of the solvent only.
- Moles of solute per litre of solution is Incorrect: Moles of solute per litre of solution is the definition of Molarity (M).
Key Takeaway
Molality is independent of temperature because it is based on mass, whereas Molarity is temperature-dependent because volume changes with temperature.