The strong polarity of water relative to hydrocarbons means that water will have a more difficult time breaking out of its liquid phase, and into its gas phase to generate a vapor pressure. Substances with a high vapor pressure generally have weaker intermolecular bonds and a lower boiling point.

In this example, methane, , has the lowest molecular weight. Hydrocarbons that have the lowest molecular weight have the least opportunity for van der Waals forces to keep them from moving into the gaseous state; thus, they have the greatest tendency to form vapor and have the greatest vapor pressure. Recognize that the greater the intermolecular forces, the higher the boiling point and lower the vapor pressure.

Sodium chloride is a solid salt. Solids do in fact have vapor pressures, but the ionic structure of this salt makes it very low.

Melting points of hydrocarbons are determined by two main factors: length of the carbon chain and degree of saturation. Longer carbon chains will have higher melting points, and chains with more saturated bonds have higher melting points.

Of the given answers, has the longest carbon chain and is fully saturated. It will thus have the highest melting point.

Polarity is determined by differences in electronegativity between the two atoms involved in a bond. A large difference in electronegativity will result in a more polar compound. Symmetry, however, can balance net polarities in bonds, can cancel the differences.

has tetrahedral geometry, and since the four groups attached to the central silicon atom are identical, this molecule has no net dipole moment due to its symmetry. It is the most non-polar compound.

In comparing  and , recall that carbon, nitrogen, and oxygen are in the same row of the periodic table, but carbon is farther from oxygen than nitrogen. This means there is a greater electronegativity difference in  than , and  is going to be more polar.

Finally, is an ionic compound, so it is going to be the most polar of all four compounds.

Of the answers, only H₂O has a net dipole moment, making water the polar molecule. All the other molecules have balanced structures and no difference in electronegativity between side groups. 

Dipole-dipole interactions are intermolecular forces that result from attraction of partial charges of atoms. Partial charges are caused by uneven sharing of electrons between atoms. For example, a covalent bond between a hydrogen and a chlorine atom will cause uneven electron sharing between the two atoms. Chlorine, a more electronegative atom, will attract the electrons closer than hydrogen; therefore, the chlorine atoms will have a partial negative charge whereas the hydrogen atom will have a partial positive charge.

Molecules containing partial charges, such as , are called dipoles. When dipoles are added into solution, the partial charges attract one another and form dipole-dipole interactions. In an  solution the partial positive charge of hydrogen from one  molecule will interact with the partial negative charge of chlorine from another  molecule and form a dipole-dipole interaction.

Remember that the differences in electronegativity doesn’t change the overall charge of the molecule. It just gives rise to partial charges that result from the relative location of electrons between the two atoms.

Dipole-dipole interactions occur between a partial positive and negative charge. Since partial charges arise from electronegativity differences, you are looking for molecules that contain atoms with different electronegativities.

Methanol molecules contain oxygen and hydrogen, which have very different electronegativities. Methanol molecules form dipole-dipole interactions between the partially positive hydrogen and the partially negative oxygen. This bond is also called a hydrogen bond. Hydrogen bonds are an extreme type of a dipole-dipole interaction.

Similarly, dichloromethane molecules contain chlorine and carbon atoms (very different electronegativities). Dichloromethane can form dipole-dipole interactions between partially negative chlorine atoms and partially positive carbon atoms.

Finally, propane contains only carbon and hydrogen, which have similar electronegativities. There are no dipole-dipole interactions in propane because there are no partial charges. The best answer is I and II.

The question is asking for intramolecular hydrogen bonds, meaning which of the following molecules will contain hydrogen bonds between the atoms within a single molecule. Hydrogen bonds exist only between a hydrogen and a nitrogen, oxygen, or flourine. Although acetone and dimethyl ether contain an oxygen that can make hydrogen bonds, the molecules themselves do not contain hydrogen bonds. These compounds form intermolecular hydrogen bonds only.

Para-Nitrophenol, similarly, will form intermolecular hydrogen bonds. The para positioning of substituents prevents them from interacting within a single molecule. Ortho-nitrophenol allows for such itneractions by having substituents on adjacent carbons. The hydrogen of the phenol and the oxygen of the nitro will form a hydrogen bond within a single molecule, therefore, ortho-nitrophenol is the only molecule present that contains intramolecular hydrogen bonds since it can form hydrogen bonds within itself. 

A hydrogen bond forms between a hydrogen bond donor (hydrogen) and a hydrogen bond acceptor (nitrogen, oxygen, or fluorine). The strength of a hydrogen bond can be determined by examining the acidity of the hydrogen and basicity of the acceptor. A hydrogen is more acidic when it is attached to a more electronegative atom. This occurs because the electronegative atom pulls the electron density towards itself, making it easy for the hydrogen to act as a leaving group (weaker bond).

A hydrogen on fluorine is the most acidic, and a hydrogen on nitrogen is the least acidic (of the hydrogen bonging possibilities). Basicity of the acceptor is also important in determining the strength of hydrogen bond. A more basic molecule will make the hydrogen bond stronger. Nitrogen forms the strongest hydrogen bonds, whereas fluorine forms the weakest hydrogen bonds.

In our case, the strongest bond will occur between the hydrogen from the hydroxyl group of methanol (most acidic donor) and the nitrogen from ammonia (most basic acceptor).

Hydrogen bonding is an intermolecular force that occurs between a hydrogen bond donor (hydrogen) and a hydrogen bond acceptor (nitrogen, oxygen, or fluorine). The bond is a result of the electromagnetic attraction of the partial positive charge on the hydrogen atom to the partial negative charge on nitrogen, oxygen, and fluorine.

To answer this question you need to look at the chemical makeup of each molecule and determine if the molecule contains the appropriate atoms. Methanol contains oxygen and hydrogen; therefore, methanol molecules can form hydrogen bonds. Dichloromethane and propane contain hydrogen, but they don’t contain nitrogen, oxygen, or fluorine; therefore, they can’t form hydrogen bonds.