Question

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

Answer 1

For 3M solution NaCl means, 3 moles of NaCl in 1 L of solvent
By, using the density we can find out the mass of the 1 L of solution. 
mass of solution= volume x density = 1000 ml (1 L) x 1.25 g/ml = 1250 g
mass of solute = mass of 3 moles of NaCl =  58.5 g/mol  x  3 mol = 175.5 g...............(molal mass of NaCl = 58.5 g/mol)
So, mass of solvent = mass of solution - mass of solute = 1250- 175.5 gm = 1074.5 gm = 1.07 Kg
So, molality = moles of solute / mass of solvent = 3 mol/ 1.07 kg  = 2.8 mol kg-1