Read the problem carefully and identify the values given for the moles of the solute and weight of the solvent.
Step 2 :
Apply the molality formula and substitute the given values in order to get the molality value.
Molality(M): $\frac{Number\ of\ moles\ of\ solute}{Weight\ of\ solvent\ in\ kg}$
Step 3 :
Check the molality values are in specified units.
If 250c.c solution contains 4g NaOH , find out the molality of the solution, if the density of the solution is 1.02g/ml.
Step 1 :From the data
Weight of solution = 250c.c $\times$ 1.02g/c.c = 255g
Hence weight of solvent = (255-4) = 251g
Step 2 :Molality = $\frac{4}{40}$ $\times$ $\frac{1000}{251}$ = 0.392.
Step 3 :The molality is expressed as 0.392 mol/kg.
Answer :The molality of the given solution is 0.392 mol/kg.
Concentrated HCl is 37% by weight, calculate the molality of the solution.
Step 1 :From the data 100g solution contains 37g HCl.
Hence weight of solvent = (100-37) = 63g
Step 2 :Molality = $\frac{W}{M}$ $\times$ $\frac{1000}{weight\ of\ solvent}$
= $\frac{37}{36.5}$ $\times$ $\frac{1000}{63}$
= $\frac{37000}{2299.5}$ = 16.09
Step 3 :The molality is expressed as 16.09 mol/kg
Answer :The molality of the given concentrated HCl solution is 16.09 mol/kg.