Experimental Procedure Lab 402. A. Preparing the Sample.

Experiment 5. Molar Mass of a Volatile Liquid

Experimental Procedure

Lab 402

A. Preparing the Sample

Prepare a boiling water bath on a for PARTA.31.Prepare the flask for sample. Clean a 125-mL Erlenmeyer

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 flask with 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 rubber band.

2. Prepare a boiling water bath. Half-fill a 500-mL beaker with water. Add one or two boiling chips to the water. Use a hot plate as a heat source. Secure a digital thermometer to measure the temperature of the water bath.

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

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. Be certain that neither the flask nor the clamp touches the beaker wall. Adjust the water level high on the neck of the flask (Figure 12.2).

2. Heat the sample to the temperature of boiling water. Gently heat water until it reaches a gentle boil. (Caution: Most unknowns are flammable) When the liquid in the flask and/or the vapors escaping from the holes in the aluminum foil are no longer visible, continue heating for another 5 minutes. Read and record the 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 the flask and allow it to cool to room temperature. Sometimes the remaining vapor in the flask condenses; that’s okay. Dry the outside of the flask and determine the mass (0.001 g, use the same balance!) of the flask, aluminum foil, 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 with PART A.2) and repeat PARTs B.1-B.3.

C. Determine the Volume and Pressure 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 the flask. Find the barometer in the laboratory. Read and Record the atmosphere pressure in atmospheres to correct number of significant figures.

D. Calculations

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

2. Determine the standard deviation and the relative standard deviation (%RSD). Refer to Appendix B and calculate the standard deviation and the % RSD for the molar mass of your unknown from your three trials.

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

The next steps

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