15.5% by mass is
equivalent 15.5 g urea in 100 g solution or 155 g urea in 1 kg solution.
we know that molality = moles solute / kg solvent
moles solute = 155 g / 60 g/mol = 2.58 moles urea
Since there are 155 g
urea in 1000g solution, hence the solvent is 845 g or 0.845 kg
So:
molality = 2.58 / 0.845 = 3.06 m
