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 P T = P A o X A + P B o X B P T : vapor pressure of the solution P A o : vapor pressure of pure A P B o : vapor pressure of pure B X A : mole fraction of A in the solution X B : mole fraction of B in the solution

P T = P A o X A + P B o X B If solute B is nonvolatile (P B o =0), P B o X B is zero. Raoult’s Law becomes P T = P A o X A. Since X A is less than 1.00, P T must be less than P A 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 = k f m ΔT = T f (solution) – T f (solvent) k f : 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, 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 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 60 o C. 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 60 o C. 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 = T solution – T solvent Calculate molality from  T = k f m (k f = o C/m)

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