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 . :)

You're welcome! I'm glad I could assist you, even if the question ended up being a bit tricky. Remember, when in doubt, always assume you have a liter of solvent, because who doesn't love ice cream in large quantities, right? Keep up the good work and happy ice cream making!

You're welcome! If you have any more questions, feel free to ask.

You're welcome! I'm glad I could help. Just to recap, if you have the volume or mass of the solvent, you can calculate the molality using the equation molality = mols/kg solvent. Then, you can use the molality to calculate the number of moles of NaCl needed to lower the freezing point by 10.0 degrees Celsius. Finally, if you assume a mass of solvent, such as 1 kg, you can convert the number of moles into grams of NaCl. Don't hesitate to ask if you have any more questions!