Overview

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Preparation of the Laboratory

   Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

   • Put on a lab coat, splash-proof safety glasses or goggles, and appropriate gloves for the chemicals you will be handling.
   • The day before the lab, place the Dumas tubes in an oven at 125 °C.
   • Two hours before the lab, remove the tubes from the oven and place them on a heat-resistant surface to cool. Distribute the tubes to each workstation once they have reached room temperature.
   • Place a barometer in an easily accessible location.
   • Set a plastic 250-mL beaker next to each analytical balance, and ensure that a box of laboratory wipes is available for each balance.
   • Set up a Bunsen burner in a fume hood, and place a striker and a test tube holder with the burner.
   • Set out the following glassware and equipment at each workstation (we suggest students work in pairs):

     1    500-mL Erlenmeyer flask

     1    500-mL Büchner flask

     1    400-mL beaker

     1    100-mL beaker

     1    Lab stand

     1    3-prong clamp

     1    Ring fixture with square of wire gauze

     1    Bunsen burner and fire striker

     2    18-in length of latex tubing

     1    18-in length of silicone tubing

     1    Thermometer and thermometer clamp

     1    Single-hole #4 rubber stopper

2. Preparation and Dispensing of Unknown Liquids
   • Label one 50-mL glass bottle and one 20-mL glass syringe as 'deionized water'.
   • Label three 50-mL brown glass bottles, three 5-mL glass syringes, and three 25-mL glass beakers as ‘1’ through ‘3’, and set them in a reserved fume hood.
   • Place a watch glass and 20-mL volumetric pipette next to each numbered bottle along with a pipetter. Set four 22-gauge, 6-in needles next to each syringe.
   • Group the glassware by label in a reserved fume hood according to their chemical compound: 1,2-dichloroethane, trichloroethylene, and 2-bromopropane.
   • Transfer at least 20 mL of each organohalide into its assigned bottle using a volumetric pipette: 1,2-dichloroethane in bottle 1, trichloroethylene in bottle 2, and 2-bromopropane in bottle 3. Cap each bottle once the liquid has been added.
   • Label a fifth 50-mL bottle, 5-mL glass syringe, and 25-mL beaker as 'acetone', and set a 22-gauge, 6-in needle next to the syringe.
   • Add about 20 mL of acetone in the appropriately labeled bottle and cap it.
   • When it is time to dispense the unknown liquids during the laboratory, pour at least 5 mL of an unknown into the matching beaker. Submerge the tip of the matching syringe in the unknown and slowly withdraw the plunger to fill the syringe to the 5-mL mark. Connect the needle and set down the syringe on a weight paper. Cover the beaker with a watch glass after filling.
   • Repeat this process for each organohalide.
   • Call up each student group one by one. Insert the needle of the syringe with their assigned unknown liquid into the long neck of the Dumas tube and dispense 0.5 mL. Withdraw the needle, taking care to avoid letting the liquid drip on the outside of the tube. The student group can now begin their trial.
   • Dispense the unknowns for the rest of the lab in this way, refilling the syringes as needed. At the end of the lab, follow the same procedure to dispense 0.5 to 1 mL of acetone into each tube for cleaning by students. The amount of acetone will depend on the amount of water left over in the tube.
   • If students need to re-weigh their empty Dumas tube in the middle of the experiment, you will flame dry the tube so that they can obtain an accurate measurement.
   • Light a Bunsen burner and hold the Dumas tube with a test tube holder, and pass the tube through the flame until it appears dry. Note: Three of the organohalides are flammable, so keep the opening of the tube away from the flame.
   • Then, hold the tube away from the flame for 1 min to allow any remaining vapor to condense. If you see liquid at the bottom of the tube after that, pass the tube through the flame again until it appears dry. Once no liquid condenses in the tube after one minute out of the flame, return the tube to the student group so that they can measure the mass.

Procedure

• Preparation of the Laboratory
• Preparation and Dispensing of Unknown Liquids

Results