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Exam 3: Melting Points

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Question 1

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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?

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(a) about ...

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Question 2

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

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Stirring two solid components together does not yield a homogenous mixture but a coarse mixture of particles, lumps, and crystal of the individual compounds. A small sample for a melting-point determination taken from the coarse mixture might be 100% of one component.

Question 3

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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.

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

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?

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.

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.)

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

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

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˚