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

• 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

• PT = PAoXA + PB

oXB

• 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

PT = PAoXA + PB

oXB

• If solute B is nonvolatile (PBo =0), PB

oXB is zero.

• Raoult’s Law becomes PT = PAoXA.

• Since XA is less than 1.00, PT must be less than PA

o.

• Note boiling-point elevation and freezing-point depression.

• These temperature changes depend on the composition of the solution.

• ΔT = k 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

• 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

• 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

• 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

• 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.

Calculations

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

• Extrapolate lines to get freezing point for each run.

• Get T values for each sample containing unknown: T = Tsolution – Tsolvent

• Calculate molality from T = kfm

(kf = -7.10 oC/m)

Moles solute = molality x kg PDB

• Average MW values from the two runs.

temoles_soluemass_solutMW =