5. Determine the molal freezing point depression constant (Kf) (in °Ckg/mol) of

Question

5. Determine the molal freezing point depression constant (Kf) (in °Ckg/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 °Ckg/mol) of water. 2MOL/KG & -17.0C

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

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

Explanation / Answer

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 °Ckg/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 °Ckg/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 °Ckg/mol

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