Wednesday
May 27, 2015

Homework Help: Chem help!!

Posted by Lindsay on Thursday, May 3, 2007 at 7:24pm.

A rock salt (NaCl), ice, and water mixutre is used to make homemade ice cream. How many grams of rock salt must be added to lower the freezing point 10.0 degrees C?

I have no idea what to do, please help!

delta T = Kf*i*m
delta T = 10.
Kf is the molal freezing point constant, which for water, is 1.86o C/m.
i = the number of particles, which for NaCl is 2.
m is the molality.
Calculate molality.
You will not be able to calculate the grams because there is no volume or mass of solvent used.
If you have the volume or grams of solvent, then molality = mols/kg solvent.

So I got the molality, but since I can't get the grams, I just stop? Is this question unanswerable?

If you want grams, yes. There is not enough information unless you wish to calculate the grams for some assumed mass of solvent, such as a liter. In that case the mass of H2O will be 1 kg and molality = mols/kg. Plug in molality, calcualte mols, and from there grams.

Ok thanks for your help . :)

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