Final answer:
The mixed melting point technique is used to determine if two organic compounds are identical by observing the melting point behavior of a mixture. Colligative properties such as freezing point depression can aid in calculating the molar mass of an unknown compound. Differences in polarity influence boiling points for compounds with similar molar masses.
Step-by-step explanation:
The concept of mixed melting point is useful in distinguishing between organic compounds with similar melting points. When two compounds are mixed together, if they are indeed the same substance, the melting point of the mixture will remain close to the true melting point of the pure substance. However, if the compounds are different, the melting point of the mixture will be depressed and broadened in range. This happens because the impurities disrupt the orderly crystal lattice of the solid, lowering its melting point and causing it to melt over a wider range of temperatures. Therefore, by observing the changes in melting point when your unknown is mixed with a compound of known identity, you can obtain evidence pointing toward whether your unknown is the same compound or a different one.
Certainly, when determining the molar mass of an unknown organic compound, colligative properties like freezing point depression can be particularly useful. A known mass of the compound is dissolved in a solvent to lower the freezing point, which is then measured. This change in temperature, along with the known K₁ values (cryoscopic constant), is used to calculate the molar mass of the solute, recognizing that larger changes in temperature suggest larger molar masses.
To predict boiling points of compounds with similar molar masses, differences in polarity must be considered to determine the strength of intermolecular forces - particularly dipole-dipole interactions. Compounds with higher degrees of polarity will generally have higher boiling points due to stronger intermolecular attractions that require more energy to overcome during the phase transition from liquid to gas.