a and Calculations: Molar Mass Determination by Depression of the Freezing Point

Question

a and Calculations: Molar Mass Determination by Depression of the Freezing Point Measured Freezing Point of Pure Water Finding the Freezing Point of a solution of Liquid Unknown Target mass of solute (Calculated based on the parameters in the instructions) Io g Unknown Actual mass of solute aod Trial I Freezing point of solution (observed) 13, I. Q3 Mass of solution Trial II Freezing point of solution Mass of solution Calculations: Trial I Trial II Freezing point depression molal molal Molality of unknown solution, m. Mass of solution Mass of solute Mass of solvent (water) mol mol Moles of solute mol glmol Molar mass of unknown (continued on following page)

Explanation / Answer

Trial-1

Mass of solute= 10.7 g, mass of solution = 131.23 gm, mass of water = 131.23- 10.7 gm=120.53gm

Molality = moles of solute/ kg of solvent

deltaTf= Kf*m, Kf= 1.86 deg.c/m

-(-0.4-0.2) = 1.86* m

Molality = 0.6/1.86 =0.322581 moles/Kg solvent

1000 gm of solvent contains 0.322581 moles of solute

120.53 gm contains 0.322581*120.53/1000 =0.0388 moles of solute

But, moles of solute= mass/molar mass

Molar mass = mass/moles = 10.7/0.0388=276 g/mole

Trial-2

Mass of solute= 10.7 g, mass of solution = 91.23 gm, mass of water = 91.23- 10.7 gm=80.53gm

Molality = moles of solute/ kg of solvent

deltaTf= Kf*m, Kf= 1.86 deg.c/m

-(-1.6-0.2) = 1.86* m

Molality = 1.8/1.86 =0.9677 moles/Kg solvent

1000 gm of solvent contains 0.9677 moles of solute

80.53 gm contains 0.9677*80.53/1000 =0.0779 moles of solute

But, moles of solute= mass/molar mass

Molar mass = mass/moles = 10.7/0.0779=137g/mole

2.

Mass of solution = 37.77 gm, mass of soild = 10.6, mass of water = 37.77-10.60 =27.17 gm

Freezing point depression = 0.2+5.5= 5.7 = kf*m

Where m =molality = 5.7/1.86=3.064moles/ kg solvent

1000 gm of solvent contains 3.06 moles solute

27.17 gm contains 27.17*3.06/1000 gm =0.08314 moles of solute

Molar mass of solute = 10.6/0.08314 =127.5 g/mole

Trial-2 mass of solution = 51.35 gm, mass of solvent = 51.35-10.60 =40.75 gm,

deltaTf= Kf*m, m= (0.2+5.1)/1.86 =2.85 moles/ kg solvent

1000 gm solvent contains 2.85 moles of solute

40.75 gm contains 40.75*2.85/1000=0.116 moles of solute

Molar mass of solute = 10.6/0.116= 91.4 g/mole

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