Relationship between molality and molarity:  

Molarity/Molality = (moles of solute/volume of solution) × (mass of solvent/moles of solute)

                       = mass of solvent (in kg)/volume of solution (in litre)

                       = mass of solvent (in grams)/volume of solution = W/V    

Let the density of the solution be d.     Unit= g mL−1     

Mass of solution = V × d  

Mass of solute = number of moles × molecular mass of solute  

                    = n mA  

Mass of solvent, W = mass of solution – mass of solute  

                     = V × d – n × mA    

Thus,         Molaity/Molality (M/m) = V×d–n×mA/V

                       Molaity (m) = molarity×V/V×d–n×mA    

                                        = molaity/d–n/V×mA    

                                    m = molaity/d–molaity×mA    

where wZ, wB are masses of solvent and solute respectively xA and xB are mole fractions of solvent and solute respectively mA is molecular mass of solvent.

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