3. In an experiment, you have isolated an unknown compound, which shows an obser
ID: 1029919 • Letter: 3
Question
3. In an experiment, you have isolated an unknown compound, which shows an observed melting range of 110-114 °C. 3.1. Which is more likely to be, X (listed mp 117-118 "C) or Y (listed mp 106-107 'C) (0.5 Point) Why might your sample not have the same melting range as either of the known compounds (given that it must be one of them)? (0.75 Point) 3.2. 4. Three test tubes contain white crystalline organic solids A, B, and C, each of which melts at 90-92°C. A 50-50 mixture of A and B melts at 75-92 "C. A mixture of 50-50 of A and C melts at 90-92 °C. In what range would a 50-50 mixture of B and C probably melt? What can you say about the identities of A, B, and C? (1.5 Points) What is the effect of small amount of impurity on both melting point and boiling point of organic compounds? (1 Point) 5.Explanation / Answer
3.1 Observed melting point range is 110-114 degree celsius. The unknown compound is more likely to be X because the presence of impurities might make it melt at a lower mp but a substance cannot melt at a higher temperature than its theoretical melting point range.
3.2 Our sample may not show the same mp range as that of its theoretical value due to presence of impurities in it. If it were a pure sample of x or y, it will melt at the same temperature as that mentioned
4. A 50-50mixture of B and C will melt over a range of 75-90 degree celsius. We can conclude that the meltingpointof A will be highest, followed by C and mp of B will be lowest.
5. Presence of impurities can lower the melting point. This is because impurities disrupt the bonding of molecules, and hence lesser energy is required to break the bonds, thus lowering the mp ( observed as depression in melting point).
However, impurities can cause the boiling point to increase since in liquid state, molecules are mobile and would rearrange themselves to achieve maximum stability ( more attraction, stronger bonds) and thus higher energy will be needed to break the bonds (observed as elevation in boiling point)
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