Question

The density of 3 M solution of NaCl is 1.25 g mL-1. Calculate molality of the solution.

NCERT Class XII
Chemistry - Main Course Book I

Chapter 1. Some Basic Conpepts of Chemistry

Answer 1

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 Na Cl 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