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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

Experiment 13

Molecular Weight Determination by Freezing Point Depression

  • 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.
  • What are colligative properties?
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.
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.
raoult s law
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
p t p a o x a p b o x b
  • 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.
Note boiling-point elevation and freezing-point depression.
  • These temperature changes depend on the composition of the solution.
  • ΔT = k m
fpd equation t k f m
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)
  • 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.
safety 2
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.
safety 3
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.
  • 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.
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.
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!
Remelt PDB; test tube clamped in bath.
  • Remove thermometer assembly.
  • Add first portion of unknown.
  • Swirl/stir/mix until unknown dissolves.
  • Replace thermometer.
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.
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.
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.
  • Graph temperature vs time; plot all three runs on one graph.
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)

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