To determine the freezing point of a known substance, naphthalene
ringstand gas source
test tube test tube clamps
Bunsen burner goggles
1. Assemble the Bunsen burner, attaching one end of the hose to the burner and the other to a gas source.
2. Assemble the ring stand so that a ring clamp is attached to the stand holding the test tube that will be used in the experiment.
3. Fill the test tube to approximately 1/8 capacity with naphthalene crystals.
4. Place the thermometer in the crystals so that it is surrounded by the naphthalene powder but not touching the sides or bottom of the test tube. Use a clamp to hold the thermometer in place.
5. Ignite the Bunsen burner and using direct heat melt the naphthalene powder until it completely turns to a liquid. When the temperature reaches approximately 90o Celsius, stop heating.
6. Observe the change in temperature from 90o to 70o Celsius, recording the temperature at regular intervals, preferably 15 seconds. This data will be used to make a chart later.
7. Once the temperature has fallen to 70o, melt the naphthalene which is now frozen to remove the thermometer. Properly dispose of the naphthalene liquid as instructed by the teacher.
Time Elapsed Temperature of Naphthalene Time Temperature
Initial (0:00) 100oC 7:00 78.5oC
0:30 97.5oC 7:15 78.3oC
1:00 93.0oC 7:30 78.3oC
1:30 89.5oC 7:45 79.0oC
2:00 86.1oC 8:00 79.0oC
2:30 84.6oC 8:15 79.0oC
2:45 82.3oC 8:30 79.0oC
3:00 81.2oC 8:45 79.0oC
3:15 81.0oC 9:00 79.0oC
3:30 80.5oC 9:15 78.5oC
3:45 80.2oC 9:30 78.1oC
4:00 80.0oC 9:45 78.0oC
4:15 79.9oC 10:00 78.0oC
4:30 79.8oC 10:15 77.5oC
4:45 79.4oC 10:30 77.0oC
5:00 79.1oC 10:45 76.5oC
5:15 79.1oC 11:00 76.0oC
5:30 79.0oC 11:15 75.2oC
5:45 78.9oC 11:30 73.8oC
6:00 78.8oC 11:45 73.0oC
6:25 78.8oC 12:00 72.1oC
6:30 78.7oC 12:15 71.1oC
6:45 78.6oC 12:30 70.3oC
Using 80.1 oC as the theoretical value for the freezing point of naphthalene, we can now
determine percent error.
Percent Error = ((Theoretical – Experimental) / Theoretical) x 100
Percent Error = ((80.1 oC – 79.0 oC) / 80.1oC) x 100
Percent Error = 1.4%
In this lab, we heated the known substance naphthalene in a test tube to approximately
100oC and observed its temperature while it cooled to approximately 70oC. Over a time period of 12 minutes and 30 seconds, we recorded the temperature at regular 15-second intervals, and, with this data, constructed a chart showing the general curve. Upon inspection of the graph and our data chart, we found the experimental freezing point of naphthalene to be around 79oC. This results in 1.4% error when compared to the actual value for the freezing point of naphthalene, 80.1oC. Considering the impurities in the consumer-grade naphthalene, the interference of outside air on the temperature of the test tube and its contents, and the inaccuracy of 1/10 measurements on a thermometer graduated by whole numbers, the error we acquired in this lab was minimal and easily explained.
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