Question

5. Determine the molal freezing point depression constant (Kf)
(in °C⋅kg/mol) of water by using the data of three NaCl solutions .
Use Equation 3 in the “Background and Procedure” file, the molality
values of all solutes from Question s 1 – 3 and the freezing point
depression ( ∆T) from Question 4. In your calculation, rewrite the
symbol“m ” (the molality) as “mol/kg” so that the molal freezing
point depression constant will have the correct unit. **Make
sure to show all your work to receive credits.**

Use the data of Solution B and determine the molal freezing
point depression constant (Kf) (in °C⋅kg/mol) of water.
**2MOL/KG & -17.0C**

Use the data of Solution C and determine the molal freezing
point depression constant (Kf) (in °C⋅kg/mol) of water.
**0.9MOL/KG & -14.OC**

Use the data of Solution D and determine the molal freezing
point depression constant (Kf) (in°C⋅kg/mol) of water**.
0.5MOL/KG & -2.0C**

Answer #1

solution B)

molality = 2 mol / kg

Tf = -17.0 oC

delta Tb = ix Kf x m

Tf - To = 2 x Kf x 2

-17 - 0 = Kf x 4

Kb = 4.25 oC / m

molal freezing point depression constant (Kf) = 4.25 °C⋅kg/mol

Solution C)

molality = 0.9 mol / kg

Tf = -14.0 oC

delta Tb = i x Kf x m

Tf - To = 2 x Kf x 0.9

- 14 = 2 x Kf x 0.9

Kb = 7.78

molal freezing point depression constant (Kf) = 7.78 °C⋅kg/mol

solution D)

molality = 0.5 mol / kg

Tf = -2.0 oC

delta Tb = i x Kf x m

Tf - To = 2 x Kf x 0.5

- 2 = 2 x Kf x 0.5

Kf = 2.0

molal freezing point depression constant (Kf) = 2.00 °C⋅kg/mol

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b. c. d. e.
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The freezing point, Tf, of a solution is lower than the
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ΔTf=Tf(solvent)−Tf(solution)
The boiling point, Tb, of a solution is higher than the
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ΔTb=Tb(solution)−Tb(solvent)
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