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 …show more content…
Melting point-mass percent composition diagram for benzoic acid and mandelic acid.
Discussion
To obtain an accurate melting point, a slow heating rate was necessary and important. Heating a sample too rapidly may have caused the thermometer reading to differ from the actual temperature of heat source. It was also difficult to measure the temperature at which the crystals first begin to liquefy due to the droplets formed from sweating. And soluble impurity usually has effects on a melting point, it causes a decrease in the melting point and it broadens the melting point range.
First part of experiment displayed a slight error in results (see table 1). A melting point range of 122.0-123.1˚C was achieved for benzoic acid which agreed with the published melting point of 122.0-123˚C. However, the mandelic acid melting point range of 117.1-118.9˚C did not agree with the published melting point data of 120-122˚C. This could have been due to the impurity of the mandelic acid used in the experiment which caused a slight shift to the melting point or the compound was heated to rapidly near the melting point.
In the second part of the experiment the melting point range for the four mixtures were recorded (see table 2). From this data a melting point-mass percent composition diagram was created (see fig. 1) and the eutectic point is crossed with the eutectic line