Final answer:
The compound with a melting point of -78.0°C would have the weakest intermolecular forces because a lower melting point indicates weaker forces are present; in this case, likely only London dispersion forces due to the nonpolar nature of the compound.
Step-by-step explanation:
The question is asking to identify which of the given compounds will have the weakest intermolecular forces based on their melting points. The compound with the lowest melting point is likely to have the weakest intermolecular forces since it requires lesser energy to change from a solid to a liquid state. Considering the provided melting points of 20.0°C, -78.0°C, and 45.0°C, the substance with the weakest intermolecular forces would be the one with the melting point of -78.0°C. This is because a lower melting point suggests that less energy is necessary to overcome the intermolecular forces, pointing to the presence of weaker forces to begin with.
In the context provided, the chemicals have similar molar masses, and the compound with the melting point of -78.0°C is likely to be nonpolar and contain only London dispersion forces, which are generally weaker compared to other types of intermolecular forces like hydrogen bonding or dipole-dipole interactions. Therefore, the presence of solely London dispersion forces contributes to this compound having the lowest melting point and thus the weakest intermolecular forces.