The compound expected to have the lowest vapor pressure among the given options is e) H2O (water).
Vapor pressure is a measure of a substance’s tendency to evaporate. It is influenced by intermolecular forces; the stronger the forces holding the molecules together, the lower the vapor pressure. In the context of the provided compounds:
- CH4 (methane): This molecule has weak Van der Waals forces since it is non-polar.
- SiH4 (silane): Similar to methane, it also exhibits weak Van der Waals forces and would have a relatively higher vapor pressure.
- NH3 (ammonia): This molecule can form hydrogen bonds, which are stronger than Van der Waals forces, leading to a lower vapor pressure than CH4 and SiH4 but higher than water.
- PH3 (phosphine): It has weaker Van der Waals forces, making its vapor pressure higher than that of NH3.
- H2O (water): Water exhibits strong hydrogen bonding due to its polar nature, significantly decreasing its vapor pressure.
The strong hydrogen bonds in water lead to a much lower tendency to evaporate compared to the other compounds listed. Therefore, H2O has the lowest vapor pressure among these options.