Final answer:
The best property of a substance to estimate its relative vapor pressure is the boiling point, as it reflects the temperature at which a substance's vapor pressure equals external pressure and is influenced by intermolecular forces.
Step-by-step explanation:
The property of a substance that is best used to estimate its relative vapor pressure is its boiling point. The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. Therefore, if a substance has a high boiling point, it indicates that it has strong intermolecular forces and typically a lower vapor pressure at a given temperature compared to a substance with a lower boiling point.
When comparing substances, the boiling point can provide insight into their relative vapor pressures. For example, in Table 22.9.1, it is indicated that substances with similar molar masses can have quite different boiling points, which is influenced by the type of intermolecular forces present. This could be hydrogen bonding, dipole-dipole interactions, or London dispersion forces, with hydrogen bonding generally resulting in the highest boiling points due to its strength.