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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


Purpose
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
Introduction

  • What are colligative properties?


Experiment 13

  • 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.


Experiment 13


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
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.


Experiment 13


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)


Safety
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.


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.


Procedure
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.


Experiment 13


Experiment 13

  • 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!


Experiment 13

  • Remelt PDB; test tube clamped in bath. bath. When PDB melts, fit thermometer-stirrer assembly. Remove test tube from bath and allow to cool. Stir continuously.

  • Remove thermometer assembly.

  • Add first portion of unknown.

  • Swirl/stir/mix until unknown dissolves.

  • Replace thermometer.


Experiment 13

  • Remove sample from water bath. bath. When PDB melts, fit thermometer-stirrer assembly. Remove test tube from bath and allow to cool. Stir continuously.

  • 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.


Experiment 13

  • Remelt sample. Remove thermometer assembly. bath. When PDB melts, fit thermometer-stirrer assembly. Remove test tube from bath and allow to cool. Stir continuously.

  • 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.


Experiment 13

  • 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.


Calculations
Calculations and pour mixture into waste container.

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


Experiment 13


Experiment 13

Moles solute = molality x kg PDB and pour mixture into waste container.

  • Average MW values from the two runs.