Problem: What mass of salt (NaCl) should you add to 1.52 L of water in an ice cream maker to make a solution that freezes at -12.4 oC ? Assume complete dissociation of the NaCl and density of 1.00 g/mL for water.

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We’re being asked to determine the mass of salt(NaCl) that must be added to 1.52 L of water to get a solution that freezes at –12.4 ˚C.


Recall that the freezing point of a solution is lower than that of the pure solvent and the change in freezing point (ΔT­f) is given by:


ΔTf=Tf, pure solvent-Tf, solution


The change in freezing point is also related to the molality of the solution:


ΔTf=imKf


where: 

i = van’t Hoff factor

m = molality of the solution (in m or mol/kg)

Kf = freezing point depression constant (in ˚C/m)


Recall that the molality of a solution is given by:


molality=moles solutekg solvent



For this problem, we need to do the following:

Step 1: Calculate for ΔTf.

Step 2: Determine the molality of the solution.

Step 3: Calculate the mass of NaCl needed.


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Problem Details

What mass of salt (NaCl) should you add to 1.52 L of water in an ice cream maker to make a solution that freezes at -12.4 oC ? Assume complete dissociation of the NaCl and density of 1.00 g/mL for water.

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Freezing Point Depression concept. If you need more Freezing Point Depression practice, you can also practice Freezing Point Depression practice problems.

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