The density of 3M solution of NaCl is 1.25 gram per ml . Calculate molality of the solution.

Molarity 3M means 3 moles of NaCl are in 1000ml or 1 L water
molality means no. of moles of solute present per kg of solvent
density of solution is 1.25 g/ml 
density= weight/volume
=>wt of solution = density x volume=1.25 ×1000 = 1250 gm
wt of NaCl is = mole x molecular weight=3×58.5 = 175.5                       [ molecular weight = 58.5]
wt of solvent = 1250-175.5= 1074.5gm= 1.0745kg
molality ==  = 2.791m