4. A student was given an unknown pure solid and was asked to determine the iden
ID: 712100 • Letter: 4
Question
4. A student was given an unknown pure solid and was asked to determine the identity of the unknown by doing a mixed melting point analysis. The student was also told that the solid was one of the six solids in the table below. The student took the melting point of his unknown and found it to be 112-113 °C. (6 points) a. Suggest which mixed melting points should the student perform? Explain b. Is there a mixed melting point that is not needed to be performed given the data? c. How would the student determine the identity of the unknown solid? Explain. Content is copyrighted. This material may not be posted to any website without permission from Dr. Koppel. Solid 2-methylbenzoic acid Anthranilamide Acetanilide Benzoquinone Benzoic acid trans Stilbene Literature melting range (°C) 105- 106 108 -110 113-115 113-115 122-123 122 -124 raiExplanation / Answer
Definition of MIXED MELTING POINT
The fusion temperature of a mixture of two components that in the case of
two different substances is usually lower than that of either component or
that in the case of a mixture of two samples of the same substance prepared
by different methods as a check on their identity is not lower than that of
either sample.
If two substances melt at the same temperature, a Mixed Melting Point determination can reveal if
they are one and the same substance.
The mixed melting point technique is an important reason why all high-quality melting point
instruments accommodate at least three capillaries in their heating blocks.
In its most common implementation, three melting points are determined: (1) sample, (2) reference and
(3) reference and sample in a 1:1 mixing ratio. If the melting point of the mixture remains the same, the
two substances are identical. If the melting point is depressed, they are two different substances.
Answer (a) The student should perform mixed M.P. analysis with first four compounds given in table because mixed M.P. is always less than equal to the M.P. of pure substance to be identified.
Answer (b) The mixed M.P. analysis with last two compounds given in table should not be performed.
Answer (c) The mixed melting point record by the student should match with the reference data given in
the table. By comparing the student will conclude that the mixed mixture is either acetanilide or
benzoquinone.
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