How many grams of antifreeze, C2H4(OH)2, would be required per .5kg of H2O to prevent the water from...?

Question

...freezing at -20 degrees celsius?

I just need to know what formula to use and how to set it up.
Don't do it for me please.

Answer 1

OK...You should have an equation in your text book that relates the freezing point depression in a solution to the molarity of the solute and to the "freezing point depression constant".

The equation should looks something like:
DTf = Kfm Where DTf is the freezing point depression (-20 degrees) and Kf is the molal freezing point depression constant (-1.86 degrees/molal for water).

Using that, you can calculate the molality of a solution which will freeze at -20. Molality is moles of solute per kilogram of water, so you can get moles/.5 kg water by dividing your answer by 2. Finally convert the moles of solute to grams solute using the molar mass of your compound.