Experimental Procedure

Lab 402

A. Preparing the Sample

Prepare a boiling water bath on afor PARTA.31.Prepare the flask for sample. Clean a 125-mLErlenmeyer flask and dry it either in a drying oven or by allowing it to air-dry. Do not wipe it dry or heat it over a direct flame. Cover the dry flaskwith a small piece of aluminium foil and secure it with a rubber band.Determine the mass (0.001 g) of the dry flask, aluminum foil, and rubberband.

2. Prepare a boiling water bath. Half-fill a 500-mLbeaker with water. Add one or two boiling chips to the water. Use a hotplate as a heat source. Secure a digital thermometer to measure thetemperature of the water bath.

3. Place the sample in the flask. Record the numberof the unknown liquid on the Report Sheet. Transfer about 5 mL of theunknown liquid into the flask; again cover the flask with aluminum foiland secure the foil with a rubber band. You do not need to conduct amass measurement. With a pin, pierce the aluminium foil severaltimes.

B. Vaporize the Sample

1. Place the flask/sample in the bath.Lower the flask/sample into the bath and secure it with a utility clamp. Becertain that neither the flask nor the clamp touches the beaker wall. Adjustthe water level high on the neck of the flask (Figure 12.2).

2. Heat the sample to the temperature ofboiling water. Gently heat water until it reaches a gentle boil.(Caution: Most unknowns are flammable) When the liquid in the flaskand/or the vapors escaping from the holes in the aluminum foil are nolonger visible, continue heating for another 5 minutes. Read and recordthe temperature of the boiling water.

The movable arm hood is a local exhaust system that can be ventilated small contaminated air to protect both user and area safety from harmful

material.

3. Measure the mass of the flask/sample. Remove theflask and allow it to cool to room temperature. Sometimes the remainingvapor in the flask condenses; that’s okay. Dry the outside of the flask anddetermine the mass (0.001 g, use the same balance!) of the flask, aluminumfoil, rubber band, and the remaining vapor.

4. do it again and again Repeat the experiment for Trial 2 and 3.You only need to transfer another 5 mL of liquid to the flask (i.e., begin withPART A.2) and repeat PARTs B.1-B.3.

C. Determine the Volume andPressure of the Vapor

1. Measure the volume of the flask. Fill the empty 125-mL Erlenmeyer flask to the brim with water. Measure the volume (0.1 mL)of the flask by transferring the water to a 50- or 100-mL graduated cylinder.Record the total volume.

2. Record the pressure of the vapor in theflask. Find the barometer in the laboratory. Read and Record theatmosphere pressure in atmospheres to correct number of significantfigures.

D. Calculations

1. Molar mass from data. Calculate the molar mass of yourunknown for each of the three trials.

2. Determine the standard deviation and therelative standard deviation (%RSD). Refer to AppendixB and calculate the standard deviation and the % RSD for the molar massof your unknown from your three trials.

3. Obtain group data. Obtain the values of molar mass for thesame unknown from other chemists. Calculate the standard deviation andthe % RSD for the molar mass of the unknown.

The next steps

A number of techniques can be used to determine the molarmass of a volatile liquid; the most common (if the instrument isavailable) is mass spectrometry. Describe how your sample’smolar mass would be determined using mass spectrometry.Search the Internet for other procedure that can be used tomeasure the molar mass of volatile substances.

Unknown Liquids

MethanolAbsolute Ethanol

Acetone2-propanoln-hexane

Cyclohexane1-pentaene

Cis-2-pentene2-butanoneN-pentane

Cyclohexene1-hexene

Ethyl acetate

DISPOSAL:Dispose of the leftover unknown

liquid in the Waste Organics container in the chemical hood.