molality is defined as the number of moles of solute in 1 kg of solvent
in this case solvent is H₂O and solute is HC₂H₃O₂
number of moles of solute - 0.500 mol
mass of solvent - 0.125 kg
since we need to know how many moles in 1 kg solvent
if 0.125 kg contains - 0.500 mol
then 1 kg contains - 0.500 mol / 0.125 kg = 4 mol
therefore molality of solution is 4 mol/kg