The density of a 3M solution of NaCl is 1.25g/ml. Calculate the molality of the solution.

3 Molar solution means  there are 3 moles of  NaCl salt in 1 Liter.  Molecular weight of NaCl = 58.44.    

Hence, there are $3\times 58.44$ gms in 1 Litre of water.

Density $=\frac{mass}{volume}$

Mass of 1 litre of solution = 1.25 gms/ml * 1000 ml = 1, 250 gms

V = volume of water added to make the solution

  175.32 gms + 1 gm /ml * V = 1, 250 gm

       $⟹V=1,074.68ml$

So 1,074.68 ml or 1, 074.68 gms of water is mixed with 3 moles of Na Cl to make the 3 M solution.  

       

   Molality = mass of solute in number of moles / mass of solvent in kg

               $=\frac{3}{1.07468}$kg

               $=2.7915$ Molal