In an experiment, an isolated unknown compound showed an observed melting range
ID: 498969 • Letter: I
Question
In an experiment, an isolated unknown compound showed an observed melting range of 120-124 degree C. Which is more likely to be, compound X (listed mp 127-128 degree C) or Y (listed mp 116-117 degree C)? Discuss your answer. An unknown white solid has melting point range 119-121 degree C. Benzoic acid is a white solid of 122 degree C or melting point. How can a student confirm the identity of the unknown solid (is it benzoic acid not) An unidentified compound is observed to melt at 111 degree C with the evolution of a gas. The sample then solidifies and does not melt until the temperature reaches 155 degree C, at which time it again melts sharply. Briefly explain these observations. What is the effect of a small amount impurity on both the melting point and the boiling point of organic compounds. Explain why?Explanation / Answer
1. Presence of impurities lowers the melting point of a compound as the number of intermolecular attractive forces between pure compound molecules decreases, so less energy is needed to break the remaining intermolecular bonds. Thus the unknown would be compound X. Purifying the material would increase the melting point and would match exactly with that of Compound X.
2. To confirm If the unknown solid is benzoic acid, the student may mix 50% benzoic acid with 50% unknown solid and rerun melting point experiment. If the melting point does not change, it is benzoic acid else, it is some other solid.
3. The unidentified compound looses its water of crystallization and melts at lower temperature initially. Now the compound is more pure and once resolidification occurs, it melts at a higher temperature. The loss of gas, makes the intermoelcular forces more strong between molecules of the compound and thus the compound melts at higher temperature.
4. Addition of small amunt of impuritiy reduces melting point of a compund as it reduces the number of intermolecular attractive forces present between the molecules. On the other hand, addition of impurities raises the boiling point of a pure compound as the impurities now need additional heat to raise its temperature to boiling.
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