In attempting to identify an unknown carboxylic acid, you determine its melting
ID: 788887 • Letter: I
Question
In attempting to identify an unknown carboxylic acid, you determine its melting point as 140-141 oC. Tables list three common carboxylic acids with melting points of 140 oC; 2-chlorobenzoic acid, 3-nitrobenzoic acid, and suberic acid. Assuming you have pure samples of these three carboxylic acids, and that the unknown carboxylic acid is one of these three, explain how you could identify the unknown compound using only melting points. Describe exactly what you would do and expect to observe.
In attempting to identify an unknown carboxylic acid, you determine its melting point as 140-141degree C. Tables list three common carboxylic acids with melting points of 140 degree C; 2-chlorobenzoic acid, 3-nitrobenzoic acid, and suberic acid. Assuming you have pure samples of these three carboxylic acids, and that the unknown carboxylic acid is one of these three, explain how you could identify the unknown compound using only melting points. Describe exactly what you would do and expect to observe.Explanation / Answer
In order to determine the identity of the carboxylic acid, mixtures must be created. In three different capillary tubes, samples of the known pure carboxylic acids should be added to an equal proportion of the unknown carboxylic acid. Upon melting, two of the mixtures should melt with an extended range, at a temperature lower that 140-141C. One of the samples should demonstrate a range of 140-141C still (or at least closer to that than the others), and this is the sample that identifies the unknown. Impurities decrease melting point, and therefore when mixing the two carboxylic acids a decrease will occur. However, one of the mixtures will still result in a pure substance, being that both of the samples are the same. This "pure substance" of mixtures will be that which demonstrates a melting point closest to that of the individual unknown sample.
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