Colligative Properties?

Question

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

Freezing point of water is 0.0 and Kf is 1.86

i just want to check my answers please.

I need the actual answer so i can check my own please.

Answer 1

delta T = kf m n

kf = 1.86

m = molality = moles / Kg solvent

n = number of ions produced for NaCl >> Na+ + Cl- >> 2

10.0 = m x 1.86 x 2

m = 2.69

if the mass of the water = 1 Kg

moles = 2.69

2.69 x 58.453 g/mol = 157.2 g

Answer 2

The freeze point depression is based on moles of particles. Since NaCl ionizes to Na and Cl, you actually have 2 moles of particles for every mole of NaCl.

The formula is delta T = mKf, where m is molality, or moles per kg. of solvent.

delta T = 10 degrees.
Kf is 1.86

Now solve for molals. You didn't specify how much water, so you can't go much further.

But lets assume per 1 kg of water. Since you have the molals, you can solve for moles of particles. Divide by 2 and you have moles of NaCl.

Finally, multiply by the molecular mass of NaCl and you will have grams. (per 1 kg. of water)