For physical measurements, the compositions of solutions, especially electrolyte solutions, are expressed in terms of molality rather than mole fractions. The development of the necessary thermodynamic equations directly in terms of molality is not common in textbooks, and the treatment in the literature is not very systematic. We develop a methodical treatment of this topic and define a new reference solution that can be used for solutions whose composition is described in terms of molality. Thermodynamic quantities per mass of solvent appear naturally in the derivation. We discuss how these quantities may be used to analyze experimental data. (Contains 1 figure.)