How much ethylene glycol (c2h6o2, the major component of antifreeze) must be added to 1 l of water to keep it from freezing at -20 oc?
kf = 1.86 oc/m.

Colligative properties calculations are used for this type of problem. Calculations are as follows:

ΔT(freezing point)  = -(Kf)m
-20.0  = -1.86 °C kg / mol (m)
m = 10.75 mol/kg

10.75 mol C2H6O2 / kg water ( 1 L water ) ( 1 kg/L ) (62.07 g/mol )= 667.25 g

Hope this answers the question. Have a nice day.