Question

a nacl solution of density of 3 M solution is 1.25 g/ml then what is the molality and mole fraction of solution

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 

Answer 2

You know that your solution has a molarity of 3 M, which implies thatevery 1 L of solution contains 3 moles of sodium chloride, the solute.

To make the calculations easier, let's pick a 1-L sample of this solution. Use the molar mass of sodium chloride to calculate how many grams of solute it contains

3moles NaCl⋅58.44 g1mole NaCl=175.32 g

Now, use the density of the solution to determine its mass.

1L solution⋅103mL1L⋅the density of the solution1.25 g1mL solution=1250 g

To find the mass of water present in the sample, subtract the mass of the solute from the mass of the solution

mwater=1250 g−175.32 g

mwater=1074.68 g

This is equivalent to

1074.68g⋅1 kg103g=1.07468 kg

So, if your solution contains 3 moles of sodium chloride for every 1.07468 kg of water, the solvent, you can say that 1 kg of water will contain

1kg water⋅3 moles NaCl1.07468kg water=2.79 moles NaCl

Therefore, the molality of the solution will be equal to

molality = 2.8 mol kg−1−−−−−−−−−−−−−−−−−−−−−