Question

Suppose you have two colorless molecular liquids, one boiling at \(-84{ }^{\circ} \mathrm{C}\), the other at \(34{ }^{\circ} \mathrm{C}\), and both at atmospheric pressure. Which of the following statements is correct? For those that are not correct, modify the statement so that it is correct. (a) The higher-boiling liquid has greater total intermolecular forces than the other. (b) The lower boiling liquid must consist of nonpolar molecules. (c) The lower-boiling liquid has a lower molecular weight than the higher-boiling liquid. (d) The two liquids have identical vapor pressures at their normal boiling points. (e) At \(34^{\circ} \mathrm{C}\) both liquids have vapor pressures of \(760 \mathrm{~mm} \mathrm{Hg}\).

Short answer

The correct statement is: (a) The higher-boiling liquid has greater total intermolecular forces than the other. Statement (d) is also correct, as the two liquids have identical vapor pressures at their normal boiling points. The other statements are incorrect but can be modified as follows: (b) The lower boiling liquid may consist of nonpolar molecules or polar molecules with weaker intermolecular forces than the higher-boiling liquid. (c) The lower-boiling liquid may have a lower molecular weight than the higher boiling liquid, but intermolecular forces have a greater influence on boiling points. (e) At \(34^{\circ} \mathrm{C}\), the higher boiling liquid has a vapor pressure of \(760 \mathrm{~mm} \mathrm{Hg}\), and the lower boiling liquid has a higher vapor pressure than \(760 \mathrm{~mm} \mathrm{Hg}\).

Step-by-step solution

Statement A

The statement says that the higher-boiling liquid has greater total intermolecular forces than the other. This statement is correct. Intermolecular forces play a significant role in determining the boiling points of substances. A substance with higher intermolecular forces will require more energy to break the bonds holding the molecules together, thus, resulting in a higher boiling point.

Statement B

The statement says that the lower boiling liquid must consist of nonpolar molecules. This statement is not necessarily correct. While it is true that nonpolar molecules generally have weaker intermolecular forces, such as London dispersion forces, leading to lower boiling points, it is not guaranteed that the lower boiling liquid must be nonpolar. It could be polar but with weaker intermolecular forces compared to the higher boiling liquid. The correct statement should be: The lower boiling liquid may consist of nonpolar molecules or polar molecules with weaker intermolecular forces than the higher-boiling liquid.

Statement C

The statement says that the lower-boiling liquid has a lower molecular weight than the higher-boiling liquid. This statement is not necessarily correct. While molecular weight can influence the boiling point, it is not the ultimate deciding factor. Intermolecular forces play a larger role in determining boiling points. The correct modification of the statement is: The lower-boiling liquid may have a lower molecular weight than the higher boiling liquid, but intermolecular forces have a greater influence on boiling points.

Statement D

The statement says that the two liquids have identical vapor pressures at their normal boiling points. This statement is correct. According to the definition of boiling point, it is the temperature at which a liquid's vapor pressure is equal to the external (atmospheric) pressure, which is the same for both liquids.

Statement E

The statement says that at \(34^{\circ} \mathrm{C}\), both liquids have vapor pressures of \(760 \mathrm{~mm} \mathrm{Hg}\). This statement is incorrect. The higher boiling liquid will have a vapor pressure of \(760 \mathrm{~mm} \mathrm{Hg}\) at its boiling point (\(34^{\circ} \mathrm{C}\)), but the lower boiling liquid will not have the same vapor pressure since its boiling point is \(-84^{\circ} \mathrm{C}\). At \(34^{\circ} \mathrm{C}\), the lower boiling liquid will have a higher vapor pressure than \(760 \mathrm{~mm} \mathrm{Hg}\). The correct statement should be: At \(34^{\circ} \mathrm{C}\), the higher boiling liquid has a vapor pressure of \(760 \mathrm{~mm} \mathrm{Hg}\), and the lower boiling liquid has a higher vapor pressure than \(760 \mathrm{~mm} \mathrm{Hg}\).