How to calculate molality with density and molarity?

Calculate molality given molarity and density?

• this problem is giving me trouble: What is the molality of NaCl in a solution that is 4.000 M NaCl, with a density of 1.44 g mL-1? Thanks!

• Answer:

  let there be 1 L of soution... molarity of NaCl = no. of moles of NaCl/volume of solution in litres 4 = no. of moles of NaCl/1 so no. of moles of NaCl = 4 now molality = no. of moles of NaCl/amount of solvent in kg we have no. of moles of NaCl (solute) ...we need amount of solvent (water) in kg now density of solution = mass of solution/volume of solution here volume of solution = 1 L = 1000 ml density = 1.44 g/ml so mass of solution = density of solution X volume of solution = 1000 X 1,44 = 1440 g now mass of solution = mass of solute + mass of solvent now solute is Nacl and we have 4 moles so mass of solute = 4 X molecular mass of Nacl = 4 X 58 = 232 g 1440 = 232 + mass of solvent mass of solvent = 1440 - 232 = 1208 g mass of solvent in k.g. = 1208/1000 = 1.208 kg molality = 4/1.208 = 3.311 moles/k.g. feel free to ask any questions