ΔTf = Kf * m Molar mass glucose = 180 g/molthe number of moles glucose:n = mass of solute / molar massn = 21.5 / 180n = 0.119 moles glucoseAmount of solvent in kg = 255/1000 = 0.255 Kg Molality = number of moles / solventm = 0.119 / 0.255 m = 0.466 moles/kgKf for water = - 1.86 ºC/*mTherefore:ΔTf = Kf * m ΔTf = (-1,86) * 0.466ΔTf = -0.86676 ºC
this is to the best of my brain power and i hope this helps with your problem