Question

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

Answer 1

3M means 3 moles in 1 litre of the solution.
as density is given we can find the mass 
mass = density * vol
Mass = 1000 * 1.25 = 1250g
mass of solute = 3mol of NaCl * 58.5 = 175.5g (molar mass of Nacl 58.5)
Mass of solvent= 1250 -175.5= 1074.5g =1.07kg
Molality = mol of solute/mass of solvent in kg= 3/1.07= 2.8 mol $kg^{-1}$

mole fraction = no.of mol of a / total moles
no of moles of solution = No.of moles of H2O = 1074.5/18 = 59.69 mol
total moles = 62.69
mole fraction = 59.69/62.69 = 0.013