Chemistry 121 Colligative Properties Lab Demonstration of Selected Calculations from Choice I Determination of Kf for Naphthalene To determine the Kf for naphthalene, we need to find the difference in the freezing point of pure naphthalene and the solution of 1,4-dichlorobenzene in naphthalene. Let’s say that we did this experiment, used 1. 00 g 1,4-dichlorobenzene in 10. 00 g naphthalene, and found that the freezing temperature of pure naphthalene was 78. 2°C, while that of the solution was 75. 4°C. This gives us a ? Tf of 78. 2°C – 75. 4°C = 2. 8°C.
Using the equation for freezing point depression and solving for Kf, we have… ?Tf = Kfmsolute Kf = ? Tf/msolute where msolute equals the molality of the solute. What is the molality of the solute? msolute=molality of solute = moles of solute/kg solvent moles of solute = 1. 00 g 1,4-DCB/146. 9 g/mol = 6. 81 x 10-3 moles 1,4-DCB kg of solvent = 10. 00 g naphthalene/1000 g/kg = 0. 01 kg solvent msolute = 6. 81 x 10-3 moles 1,4-DCB/0. 01 kg naphthalene = 0. 681 m Kf = 2. 8°C/0. 681 m = 4. 112 K·kg/mol The actual value for Kf for naphthalene is 7. 45 K·kg/mol, so we’re a fair amount off the mark.
This is a rather crude experiment, so that’s to be expected. Determination of the Molecular Mass/Molecular Formula of Elemental Sulfur Although this experiment didn’t go as well as I would have liked, there seemed to be some problems with the interpretation of the data as they were obtained, so I thought I’d demonstrate how to do this calculation. Elemental sulfur has a molecular formula of S8 (there are 8 sulfur atoms in a molecule of sulfur, just like there are 2 hydrogen atoms in a molecule of hydrogen). So, the purpose of this experiment was really just to see how close you could come to this answer.