Molality

Molality b gives the amount of solute per kilogram of solvent: b=n/m_solvent. The denominator is the mass of the pure solvent, not the mass of the entire solution. Dissolving 0.50 mol NaCl in 1.00 kg water gives a formal molality of 0.50 mol/kg. For particle counts in colligative properties, the degree of dissociation may also matter. The unit mol/kg remains useful even when solution volume changes with temperature. State which component counts as the solvent, and convert its mass to kilograms before dividing. In a mixture with several solutes, report molality separately for each component.

Molality expresses composition without relying on a temperature-dependent volume measurement. Volumes expand on heating, while mass stays effectively constant in an ordinary experiment. This makes b useful for freezing-point depression and boiling-point elevation. The denominator is the mass of solvent before or without the solute. In 100 g water, that is 0.100 kg; dissolving 0.20 mol gives b=2.0 mol/kg. Using total solution mass instead calculates a different quantity. State which component counts as the solvent, and convert its mass to kilograms before dividing. In a mixture with several solutes, report molality separately for each component.

Molality is amount per mass of solvent in mol/kg. Amount concentration c is amount per volume of solution in mol/L. They are not identical because solvent and solution volumes differ, and density is needed for conversion. In dilute aqueous solutions near room temperature their numerical values may sometimes be similar, but that is an approximation, not a definition. Use the quantity required by the equation and source, and label the denominator explicitly. State which component counts as the solvent, and convert its mass to kilograms before dividing. In a mixture with several solutes, report molality separately for each component.

First convert solvent mass to kilograms and determine the amount of dissolved substance. Example: 0.10 mol glucose in 250 g water. The solvent mass is 0.250 kg, so b=0.10/0.250=0.40 mol/kg. A reasonableness check helps: for the same amount of solute, less solvent gives higher molality. Do not add solute mass to the denominator. If amount must first be found from a mass in grams, use n=m/M with the correct molar mass. State which component counts as the solvent, and convert its mass to kilograms before dividing. In a mixture with several solutes, report molality separately for each component.

In a closed sample, heating normally does not change the amounts or masses of solvent and solute. Since molality uses only those quantities, b usually remains constant. A volume concentration can change through thermal expansion even if no material is added or lost. Exceptions occur if a substance escapes, precipitates, reacts or water evaporates. Precise thermodynamic calculations must also consider activity and non-ideality; constant molality does not automatically guarantee ideal solution behavior. State which component counts as the solvent, and convert its mass to kilograms before dividing. In a mixture with several solutes, report molality separately for each component.