Molality (molal concentration) is the substance amount in the weight of the solvent (eg mol/kg). Unlike molarity, it is the ratio of mass of solute to mass of solvent (without solute). For very dilute aqueous solutions, the value of molality and molarity hardly differ, because 1 L of water is approximately equal to 1 kg of water and the amount of dissolved substances can be neglected. For more concentrated solutions, where the volume of dissolved substances contributes significantly to the total volume of the solution, the molality of the solution is less than its molarity for the same amount of substance.
ŠVÍGLEROVÁ, Jitka. Molality [online]. [cit. 2010-11-10]. <https://web.archive.org/web/20160416171954/http://wiki.lfp-studium.cz/index.php/Molalita>.