Molality Calculator

Molality (m) = moles of solute / kilograms of solvent. Molarity (M) = moles of solute / liters of solution. The key difference: molality uses mass of solvent (not volume of solution) and does not change with temperature. Molarity changes slightly when solutions expand or contract with temperature. This is why molality is preferred for colligative property calculations.

Dissolving a non-volatile solute raises the boiling point of a solvent by ΔTb = i × Kb × m, where i is the van't Hoff factor (number of particles per formula unit), Kb is the ebullioscopic constant of the solvent (0.512 degrees C kg/mol for water), and m is molality. Adding 1 mol of NaCl (i=2) to 1 kg of water raises the boiling point by about 1.02 degrees C.

The van't Hoff factor (i) accounts for the number of particles a solute dissociates into. Non-electrolytes like glucose: i = 1. Strong electrolytes: NaCl = 2 (Na+ and Cl-), MgCl2 = 3, AlCl3 = 4. Ion pairing at higher concentrations causes i to be slightly less than the theoretical value. For colligative properties, all particles count equally regardless of charge.

Those are the other two colligative properties. Osmotic pressure follows Π = i·M·R·T, where M is molarity (mol/L), R = 0.08206 L·atm/(mol·K), and T is absolute temperature - a 0.1 M glucose solution at 25°C exerts about 2.4 atm. Vapor pressure lowering follows Raoult's law, P_solution = x_solvent × P°_solvent, so the drop ΔP is proportional to the mole fraction of dissolved particles. This calculator computes boiling point elevation and freezing point depression directly; osmotic pressure conventionally uses molarity (not molality) because it is measured at a single fixed temperature.