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The density of 3M solution of a sodium chloride is 1.252g/mL. The molality of the solution will be (molar mass of NaCl=58.5g/mL)

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3 Molar solution means there are 3 moles of  Na Cl salt in 1 Liter.  Molecular weight of Na Cl = 58.44.    Hence, there are 3 * 58.44 gms in 1 Litre of water.

Density = 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, 074.68 ml  

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

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

               =  3 / 1.07468 kg

               =  2.7915  Molal 

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