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Experiment 13

Experiment 13. Molecular Weight Determination by Freezing Point Depression. Purpose. The purpose of this experiment is to use the freezing-point depression of a solution of an unknown compound in para-dichlorobenzene (PDB) to determine the molecular weight of the unknown. Introduction.

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Experiment 13

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  1. Experiment 13 Molecular Weight Determination by Freezing Point Depression

  2. Purpose • The purpose of this experiment is to use the freezing-point depression of a solution of an unknown compound in para-dichlorobenzene (PDB) to determine the molecular weight of the unknown.

  3. Introduction • What are colligative properties?

  4. Examples of colligative properties Vapor pressure over a solution Boiling-point elevation Freezing-point depression Osmotic pressure • The first three colligative properties are closely related.

  5. Situation: solute B dissolved in solvent A. • If the solution is ideal (not all are!), the vapor pressure over the solution follows Raoult’s Law.

  6. Raoult’s Law • PT = PAoXA + PBoXB • PT: vapor pressure of the solution • PAo: vapor pressure of pure A • PBo: vapor pressure of pure B • XA: mole fraction of A in the solution • XB: mole fraction of B in the solution

  7. PT = PAoXA + PBoXB • If solute B is nonvolatile (PBo =0), PBoXB is zero. • Raoult’s Law becomes PT = PAoXA. • Since XA is less than 1.00, PT must be less than PAo.

  8. Note boiling-point elevation and freezing-point depression. • These temperature changes depend on the composition of the solution. • ΔT = k m

  9. FPD equation ΔT = kf m • ΔT = Tf(solution) – Tf(solvent) • kf: freezing-point depression constant for the solvent (-7.10 o/m for PDB) • m: the molality of solute (moles solute/ kg solvent)

  10. Safety • Aprons and safety glasses • Bunsen burners: keep hair, clothes, paper away. • PDB and unknowns are not soluble in water; they are also flammable and/or toxic. USE WASTE BOTTLES, NOT THE SINK, FOR DISPOSAL.

  11. Safety 2 • Thermometers are fragile and expensive; handle with care! • Do not attempt to remove the thermometer from a solidified sample. Remelt sample, then remove thermometer.

  12. Safety 3 • After experiment is over: remelt sample; remove thermometer and stirrer; pour molten sample into waste container. Then use a small amount of acetone to rinse any remaining material in test tube into waste container.

  13. Procedure • Work in pairs. • Needed equipment: Ring stand, clamp, ring, wire gauze, Bunsen burner, 1000-mL beaker, largest test tube (25 x 200 mm). • Check out thermometer-stirrer-stopper assembly from stockroom.

  14. Weigh test tube to nearest 0.01 g; record mass on data sheet. Add 20-24 g of PDB to test tube; weigh again and record mass on data sheet. • Weigh two portions of unknown, each about 1.5 grams (to nearest 0.01 g). Record masses and unknown number on data sheet.

  15. Set up water bath and burner (p. 13-6); clamp test tube in bath. When PDB melts, fit thermometer-stirrer assembly. Remove test tube from bath and allow to cool. Stir continuously. • Begin measuring when temperature reaches 60oC. Record temperature every 30 seconds. Note when solid first appears. After 8 minutes, stop collecting data. • Do not attempt to remove thermometer now!

  16. Remelt PDB; test tube clamped in bath. • Remove thermometer assembly. • Add first portion of unknown. • Swirl/stir/mix until unknown dissolves. • Replace thermometer.

  17. Remove sample from water bath. • Begin recording temperature as before when solution has cooled to about 60oC. • Note temperature at which solid first appears. • Stop collecting data after eight minutes.

  18. Remelt sample. Remove thermometer assembly. • Add second portion of unknown (total mass of solute in this run is the sum of the two individual masses). • Dissolve unknown. • Cool and collect data as before.

  19. After last run, remelt mixture, remove thermometer assembly, and pour mixture into waste container. • Use small volume of acetone to rinse remaining material from test tube into waste bottle. • Rinse thermometer assembly with acetone (into the waste bottle) and return thermometer assembly to stockroom.

  20. Calculations • Graph temperature vs time; plot all three runs on one graph.

  21. Extrapolate lines to get freezing point for each run. • Get DT values for each sample containing unknown: DT = Tsolution – Tsolvent • Calculate molality from DT = kfm (kf = -7.10 oC/m)

  22. Moles solute = molality x kg PDB • Average MW values from the two runs.

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