The molality of a given solution (m) is defined as the number of moles of the solute present per kg of the solvent. If 'n' moles of the solute are dissolved in 'a' kg of the solvent the molality of the solution. Solutions of a particular molality are prepared by dissolving the required number of grams of solute in the required number of kilograms of solvent.
The fundamental difference between molality and all the other units of concentration is that the amount of solvent is measured, and not the amount of solution. Thus the solutions are not prepared by diluting with the solvent to the specified volume of solution.