What mass of ethylene glycol (C2H6O2, molecular weight 62.1 g) must be added to 20.00 L of water to produce a solution for use in a car's radiator that freezes at -15 degrees centigrade?

(The density of water is 1 Kg L^-1. The molal freezing point depression constant for water, Kf, is 1.86 degrees centigrade Kg mol^-1)

Question

What mass of ethylene glycol (C2H6O2, molecular weight 62.1 g) must be added to 20.00 L of water to produce a solution for use in a car's radiator that freezes at -15 degrees centigrade?

(The density of water is 1 Kg L^-1. The molal freezing point depression constant for water, Kf, is 1.86 degrees centigrade Kg mol^-1)

anonymous · Answer

Students usually find it easier to solve this question in steps.  The steps are:
Step 1:  Find the molality of the solution using the freezing point.   Δtf = i  Kf m
Step 2:  Determine the moles of the solute using the molality equation. 
 molality = "moles of solute" /"kilograms of solvent" 

Step 3:  Convert moles of ethylene glycol to grams.