Molarity (M) = mol solute / L solution (temperature-dependent). Molality (m) = mol solute / kg solvent (temperature-independent — preferred for colligative properties). Mole fraction (x) = mol component / total mol (dimensionless). Key interconversion: if density (d) of solution is known and M is molarity, then m = M x 1000 / (1000d - M x $M_{solute}$). For dilute aqueous solutions: m ≈ M (since density ≈ 1 g/mL and solute mass is small). Mole fraction of solute to molality: $x_{solute}$ = m x $M_{solvent}$ / 1000 (for dilute solutions). Always use molality for colligative property calculations ($delta_{Tb}$, $delta_{Tf}$) and molarity for osmotic pressure (pi = iMRT).