Molality, molarity and colligative properties
Why molality uses solvent mass not solution volume, how it stays fixed with temperature, and where it drives freezing and boiling point shifts.
Molality versus molarity
Molality and molarity sound alike but divide the moles of solute by different things. Molality (mol/kg) divides by the mass of the solvent in kilograms, while molarity (mol/L) divides by the volume of the whole solution in litres. That single difference matters because mass is conserved when you heat or cool a beaker, but volume is not. For dilute aqueous solutions the two values sit close together, since one litre of water weighs roughly one kilogram, yet they separate as concentration rises or when the solvent is denser or lighter than water.
Why molality ignores temperature
A solution expands as it warms, so a fixed number of moles spreads through a larger volume and the molarity falls even though nothing was added or removed. Molality escapes this trap entirely because it is built on the mass of the solvent, and mass does not change with temperature. This is why lab work that spans a range of temperatures, such as boiling point or freezing point experiments, is reported in molality. When you need a concentration that stays honest from an ice bath to a hot plate, molality is the safer unit.
Colligative properties run on molality
Colligative properties depend on how many solute particles are present, not on what they are, and the standard equations are written in molality. Freezing point depression is delta T = i times Kf times b, and boiling point elevation is delta T = i times Kb times b, where b is the molality, Kf and Kb are solvent constants, and i is the van't Hoff factor counting the particles each formula unit releases. Water has a Kf near 1.86 degrees Celsius per mol/kg, so a 1 mol/kg solution of a non-dissociating solute lowers the freezing point by about 1.86 degrees. Salts push harder because they split into several ions, raising i.
Working from a mass and molar mass
Most problems hand you grams rather than moles, so the first step is to convert. Divide the solute mass in grams by its molar mass in grams per mole to get moles, then divide by the solvent mass in kilograms to reach molality. The Mass + molar mass option folds both steps together: enter 40 g of sodium hydroxide with a molar mass of 40.00 g/mol dissolved in 0.5 kg of water, and the tool reports 1 mole of solute and a molality of 2 mol/kg. Remember to weigh the solvent by itself, because using the total solution mass would understate the concentration.