Expt #1 - Melting Point Determinations

General Notes about this Lab

For this lab only, each student will work individually rather than in a team.

Reading Assignment - Mohrig Chapter 14

Experimental Procedures

A Mel-Temp apparatus (Mohrig, Fig 14.2, p 177) is to be used for all melting point determinations. Use a thermometer out of your supply drawer and remember to return it there before leaving the lab. Make sure the thermometer fits properly in the Mel-Temp. It should easily slide in the hole on the apparatus - don't force it! It should also insert fully in the hole and the bulb should be visibly resting at the same depth as the capillary tubes. 

The Mel-Temp apparatus allows the simultaneous determination of up to three mps. To save time, you should take advantage of this feature.

Lab Notes - Be careful to record observations on each material measured and how it behaves during the heating and melting processes.

Melting Point Determinations:

1. Pure Known Compounds. Obtain the mps of commercial samples of urea and cinnamic acid (CA). (We will assume that these commercial samples are relatively pure.) Use literature values of the mps of these compounds as a guide for how high you can raise the temperature quickly before slowing down to around 2 deg/min. (Recording temperature vs time data us the best way to insure and document that the rate of increase was appropriate.)

2. Mixtures. As an example of an impure substance (i.e., a mixture) we will examine the mp behavior of a 1:1 mixture of urea and cinnamic acid. Prepare the mixture by weighing out a small amount (20-50 mg) of each compound and then grind them together thoroughly using a spatula and a watch glass. Do two determinations of the mp, one fast to get the approximate starting temperature and one careful determination at a rate of about 2 deg/min.

3. Unknown Determination. Obtain the mp of the unknown provided to you by the instructor. (Again here, it is a good idea to do a quick determination first to find the approximate melting point range, then do a second more careful trial.) Carry out mixture mp tests in order to conclusively identify the unknown from among the list of possible unknowns given in the table below. Prepare your mixtures with an approximately 1:1 mass ratio. Grind all mixtures thoroughly before testing the mp.

3. Waste disposal. All solid wastes from this experiment will be collected in one of the two waste beakers in the dispensing hood. One beaker will be labeled for waste urea/cinnamic acid. The other will be labeled for waste unknown.

Here are some suggestions for things to include in your Discussion section:

• Comment on the mp behavior of pure compounds versus those of mixtures. Use the results you obtained to illustrate each conclusion.
  
• State your conclusion as to the identity of the unknown and explain the reasoning.