What is the molality of a solution that contains .500 mol hc2h3o2 and 0.125 kg h2o?

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

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