Answer:
6.8 ºC m⁻¹
Explanation:
The strategy here is to use the equation for the freezing point depression
ΔTf = Kf x m ⇒ Kf = ΔTf / m
where k is the freezing point depression constant,ΔTf is the change in the freezing point of the solution ( freezing point pure solvent minus freezing point of the solution), and m is the molality, mol of solute per kilogram solvent.
Thus
ΔTf (ºC) = -8.7 ºC - ( -13.7 )C = 5.0 ºC
mol (NH₂)₂CO = mass / molar mass = 24.17 g / 60.06 gmol⁻¹ = 0.40 mol
m = 0.40 mol / 0.550 Kg = 0.73 m
Kf = ΔTf / m = 5.0 ºC / 0.73 m = 6.8 ºCm⁻¹
