Measuring the Melting Points of Compounds and Mixtures

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TECH0701: Measuring the Melting Points of Compounds and Mixtures Introduction
This exercise dealt with the melting points of pure mandelic acid and benzoic acid. The eutectic temperature and composition of mandelic and benzoic acid mixtures were determined. And finally, an unknown was identified by its mixtures and melting point. The melting point of a compound is used by organic chemists not only to identify the compound, but also to establish its purity. To determine the melting point two temperatures were noted. The first was the point at which the first drop of liquid formed among the crystals; the second was the point at which the whole mass of crystals turned to a clear liquid. And the melting point was recorded from this range of melting (Pavia, 2005). The melting point indicates purity in two ways; the purer the material, the higher its melting point. And, the purer the material, the narrower its melting-point range. Adding successive amounts of an impurity to a pure substance generally causes its melting point to decrease in proportion to the amount of impurity (LeFevre, 2009). Methods

Measuring melting points of benzoic acid and mandelic acid. Capillary tubes were loaded with benzoic acid and mandelic acid powder at about 1-2 mm. Tubes were placed inside the melting apparatus to be heated. Since pure benzoic acid melts at 122-123˚C, the tube was rapidly heated to 110˚C and heated slowly to 1-2˚C per minute till all crystals liquefied. And the same applied to the mandelic acid capillary tube. Determining the eutectic temperature and composition of a benzoic acid-mandelic acid mixture. Four capillary tubes for the four mixtures were labeled. Mixtures 1 and 2 were placed inside the melting apparatus and were rapidly heated to 60˚C and then heated 1-2˚C per minute. The temperatures at which the crystals first started to liquefy and all samples turning liquid were recorded. The melting apparatus was then allowed to cool back down before...
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