Melting Points and Mixture Melting Points Hayley Williams, willi553@go.stockton.edu CHEM 2125 007 1/30/2018 Abstract In this experiment, un unknown solid was identified by using a Thomas Hoover Apparatus to determine the mixture melting point. The melting point (mp) range for Unknown #3 was 109.5C-112.7C. The mp range was compared to the known mp for a set of solids provided in a data table. The closest matches to Unknown #3 (acetanilide, 113-115C; fluorene, 114-116C) were mixed with the unknown substance to find mixture mp. The results concluded that the unknown was acetanilide because the melting point measured was within the literature mp range.  Purpose/Theory The goal of this experiment was to establish a correlation between mixture melting points and the purity of a substance. All solids have a tight range of melting point temperatures which can be used to identify the solid. A solid can be considered pure if, when mixed with a known solid, its melting point falls within the known range. If the melting point falls significantly outside of the known range, it is likely that the substance is not pure. In this experiment, an unknown was determined by mixture melting points. A Thomas Hoover Apparatus was used to measure melting points of both the unknown substances and the mixtures. Of the two mixture melting points observed, one would measure to be within the known range and the other would be significantly lower, proving its impurity. Results and Discussion Observed mp (C) Literature mp (C) Unknown #3 109.5-112.3 --- acetanilide 114.5-117.0 113-115 fluorene 89.2-91.5 114-116 The data presented in the above table shows that unknown #3 is acetanilide. When unknown #3 was combined with acetanilide, the observed melting point range was 114.5-117.0C which is nearly identical to the literature melting point(113-115C), exemplifying the pureness of the acetanilide. When the unknown #3 was combined with fluorene, the mixture melting point (89.2-91.5C) was much lower than the literature melting point of fluorene (114-116). The decreased melting point shows that the fluorene mixture is, in fact, a mixture as opposed to being a pure substance. Conclusions The unknown #3 is acetanilide. This was concluded by using mixture melting point methodology. The unknown was determined by mixing with two known substances and finding the melting points of each. Impure compounds melt at temperatures much lower than their literature melting point, just as the fluorene mixture did. The acetanilide mixture melting point fell into the literature melting point range, proving the compound to be pure. References Wohlrab, Aaron., Sample Lab Report Format. (2018) Zubrick, James. The Organic Chem Lab Survival Manual, 9th ed.; Wiley: NJ, 2014. Post-Lab Questions 1. In the following table, you will find the name, observed melting point and literature melting points of several substances. Based on these data, what would you say about the purity of these substances? a) Naphthalene? b) p-anisic acid? c) 3-chlorobenzoic acid? d) Ferrocene? Substance Observed Melting Point C Literature Melting Point C Naphthalene 79-80 80.5 p-anisic acid 178-182 184 3-chlorobenzoic acid 157-158 158 Ferrocene 157.5-161.5 172.5 Based on the data, naphthalene, p-anisic acid, and 3-chlorobenzoic acid are all very pure, while ferrocene is not pure because the observed melting point is significantly lower than the literature melting point. 2. Ibuprofen has a literature melting point of 75-77 °C. A student carried out a synthesis in which she hoped to obtain ibuprofen. She isolated a product having a melting range of 75-77°C. a) Was her product definitely ibuprofen? No, her product was not definitely ibuprofen. b) If the answer to the above is no (and it is!), using only the techniques of melting points, how could she prove that she had ibuprofen? To prove she had ibuprofen, she would have to mix a small amount of her substance with ibuprofen and then find the melting point. If it is ibuprofen, the melting point will remain the same. If it is not, the melting point will be much lower proving its impurity. 5. Why is it important to pack the capillary well when determining the melting point of the samples? If the sample is packed too loose in the capillary, it will heat unevenly. 6. A student was given an unknown compound X, which is one of the four compounds listed below. Phenyl succinate 121C O-toluic acid 102C m- aminophenol 122C benzoic acid 122C A mixture of X with benzoic acid melts at 92 o C, a mixture of X with phenyl succinate melts at 120 o C and a mixture of X with m-amino phenol melts at 102 o C. The student wrote that the unknown compound is O-toluic acid. Was the student right? Explain your reasoning and provide an alternate explanation if the student is wrong. Compound X is not o-toluic acid. The unknown compound is phenyl succinate. This is because, when mixed, the melting point did not differ from the literature melting point, therefore it is pure phenyl succinate.