Deck 3: Melting Points

Question 1

Look up the melting points of the following compounds in a suitable handbook, Internet site, or online source. (See Technique 17, p. 198.) Include the name of your source with your answer.
(a) Quinine
(b) Indophenol

Answer 1

(a) 177 (dec) (www.chemfinder.com)
(b) 160˚ (CRC, 70 th edition )

Question 2

What setting should you have on the Mel-Temp apparatus when you are in the vicinity of the melting point for each of the compounds in Problem 2.1?

Answer 2

(a) about 4.4
(b) about 4.1

Question 3

What setting should you have on the Fisher-Johns apparatus when you are in the vicinity of the melting point for each of the compounds in Problem 2.1?
(a) about 70
(b) about 62

Answer 3

The Fisher-Johns apparatus should be set close to the expected melting point for each compound. Therefore, for a compound with a melting point of about 70, the apparatus should be set slightly below, around 65 to allow for a gradual increase in temperature. Similarly, for a compound with a melting point of about 62, the apparatus should be set slightly below, around 57. This allows for accurate determination of the melting point without rapidly surpassing it.

Question 4

Using the following table, draw a melting-point diagram.
(a) Estimate the eutectic temperature and composition.
(b) If you wished to obtain a more accurate graph for this melting-point curve, suggest the best mixtures for additional mixed melting points.

Question 5

Why is it not good practice to mix two components by simply stirring them together and then sampling them for a mixed melting point?

Question 6

You think that you have isolated aspirin (acetylsalicylic acid) in the lab. It melts at 75˚-77˚C. Since you don't totally trust your own laboratory techniques, you want to prove to yourself that you have aspirin. Using only melting-point techniques, explain how you can prove that you actually have aspirin. (Assume the stockroom is able to supply you with any compound you need.)

Question 7

The following table lists the observed melting points for several compounds isolated in the student laboratory.
(a) Look up the melting point for each compound in a suitable resource.
(b) Which compounds are pure? Which are impure?

Question 8

Suppose that you are taking a melting point and the compound disappears: What happened? Why did it do this? What should you do?

Question 9

You and your lab partner take melting points of the same sample. You observe a melting point of 101˚-107˚C, while your partner observes a value of 110˚-112˚C. Explain how two different melting point values can be observed for the exact same sample.

Question 10

Using Table 2.1, construct a thermometer calibration graph from the following observed melt- ing points.
Acetanilide, mp 113˚
Benzamide, mp 130˚
Succinic acid, mp 188˚
p- Nitrobenzoic acid, mp 245˚