How much ethylene glycol (C2H6O2, the major component of antifreeze) must be added to 1 L of water to keep from it freezing at -15 °F?

Answer 1

Change -15F to degrees C. That's approximately -26
Then delta T = Kf*m
approx 26 = 1.86*m
solve for molality.
m = mols/kg solvent = mols/1kg
You have m and kg, solve for mols.
Then mol = g/molar mass. You have molar mass and mols, solve for g

Answer 2

14 mols/kg