Hey there!:
ΔTf = Kf * m
Molar mass glucose = 180 g/mol
number of moles glucose:
n = mass of solute / molar mass
n = 21.5 / 180
n = 0.119 moles glucose
Amount of solvent in kg = 255/1000 = 0.255 Kg
Molality = number of moles / solvent
m = 0.119 / 0.255
m = 0.466 moles/kg
Kf for water = - 1.86 ºC/*m
Therefore:
ΔTf = Kf * m
ΔTf = (-1,86) * 0.466
ΔTf = -0.86676 ºC
hope this helps!
