Chemistry Practice Problems Freezing Point Depression Practice Problems Solution: What mass of salt (NaCl) should you add to 1.52 L ...

Solution: 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.

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.

Solution

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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