Standard Compound Literature Melting Point (°C) Benzophenone 48.1 Naphthalene 80.25 A = Adipic Acid 152-153°C B = Benzilic Acid 155-156°C C = Salicylic Acid 158-159°C Melting Point Naphthalene: 77.5-79.5°C Melting Point Benzophenone: 48.0-49.0°C Melting Point Unknown: 150.5-151.5°C Appearance of Unknown: White crystalline powder Appearance of Compound A: White crystalline solid Appearance of Compound B: White powder Appearance of Compound C: White crystalline powder Melting Point Mixture (capillary 1): Unknown and salicylic acid: 148.0- 150.0°C Melting Point Mixture (capillary 2): Unknown and Benzilic acid: 120.0- 142.0°C While discussing your results, these questions should be answered. 1) Discuss the results of the calibration. Quote both the resultant melting points and the literature melting points when making comparisons. 2) What was the identity of your unknown? 3) What evidence did you use to ascertain the identity of your unknown? Include descriptions of the appearance of the relevant compounds and the results of the mixed melting points. 4) What is the usual result of a mixed melting point of two different compounds?
The Correct Answer and Explanation is:
Discussion of Results and Identification of Unknown Compound
- Calibration Results:
The melting points of standard compounds were measured to verify the accuracy of the apparatus. Naphthalene was recorded at 77.5–79.5°C, compared to its literature value of 80.25°C. Benzophenone melted at 48.0–49.0°C, closely matching the literature value of 48.1°C. These results confirm that the equipment was well-calibrated, with deviations of less than 2°C. Such minor differences are within acceptable experimental error and indicate the melting point determinations are reliable for identifying unknowns. - Identity of the Unknown:
The unknown compound was identified as Adipic Acid (Compound A). - Evidence Supporting Identification:
The unknown was a white crystalline powder, consistent in appearance with Adipic Acid, described as a white crystalline solid. The unknown’s melting point was 150.5–151.5°C, closely matching the literature range for Adipic Acid (152–153°C). To further confirm, mixed melting point tests were conducted. A mixture of the unknown and Salicylic Acid (Compound C) melted at 148.0–150.0°C, showing a depression and broadening of the melting range compared to both pure substances. This result indicates they are different compounds. The mixture with Benzilic Acid (Compound B) melted at 120.0–142.0°C, displaying a significant depression and very broad range, further confirming dissimilarity. No mixed melting point test was performed with Adipic Acid, but the unknown’s pure melting range (150.5–151.5°C) closely aligns with Adipic Acid’s literature range, supporting the identification. - Mixed Melting Points of Different Compounds:
When two different compounds are mixed, the resulting melting point is typically lower and broader than that of either pure substance. This depression occurs because the mixture disrupts the orderly crystal lattice of each pure compound, requiring less energy (lower temperature) to melt the mixture. This principle is useful for distinguishing substances with similar appearances or melting ranges.
