Questions 1. A student performs two melting-point determinations on a crystallin
ID: 540997 • Letter: Q
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Questions 1. A student performs two melting-point determinations on a crystalline product. In one determination, the capillary tube contains a sample about 1-2 mm in height and the melting range is found to be 141°- 142°C. In the other determination, the sample height is 4-5 mm and the melting range is found to be 141°-145°C. Explain the broader melting-point range observed for the second sample. The reported melt- ing point for the compound is 143°C 2. Another student reports a melting range of 136°-138°C for the compound in Question 1 and mentions in her noteboolk that the rate of heating was about 12° per minute. NMR analysis of this student's product does not reveal any impurities. Explain the low melting point.Explanation / Answer
1)
The temperature at which a solid melts is known as the melting point of the substance. If the compound melts over a very narrow range, it can be assumed pure. Consequently, compounds that melt over a wide range are assumed to be relatively impure.
The height of sample we take in the capillary tube also results wide range of melting point. The height of sample in the capillary should be 2-3 mm. too much sample will result in poor results. Melting range of 2o or less will be considered to be pure. 4o or more will consider being impure.
In one determination the height of capillary tube is 1-2 mm and the melting range is 141o-142o. It indicates that the sample is pure. In another determination the height of capillary tube used is 4-5mm and the melting point range is 141o-145o. Broader melting point range in the second sample indicates that the sample is impure.
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