Vapour pressure trend:\nFor liquids of similar chemical nature at a fixed temperature, how does vapour pressure vary with increasing molecular weight?

Introduction / Context:
Trends in vapour pressure are essential for predicting volatility, boiling points, and separation difficulty. For homologous or chemically similar liquids, intermolecular forces scale with size and structure. This question probes your understanding of the inverse relation between molecular weight and vapour pressure within a related chemical family at the same temperature.

Given Data / Assumptions:

• Liquids are chemically similar (e.g., n-alkanes).
• Temperature is fixed.
• No association/strong hydrogen bonding anomalies are introduced.

Concept / Approach:
Heavier molecules typically have stronger dispersion forces and larger surface areas, making their molecules harder to escape into the vapour phase. Thus, at a given temperature, heavier members exhibit lower equilibrium vapour pressures. This explains why boiling points increase with molecular weight in a series like C5–C10 alkanes.

Step-by-Step Solution:
Consider a homologous series (e.g., n-alkanes).As molecular weight increases, cohesive forces increase.Higher cohesion lowers vapour pressure at the same temperature.Therefore, select “decreases.”

Verification / Alternative check:
Antoine constants and vapour–pressure tables confirm declining vapour pressure with increasing carbon number at fixed temperature.

Why Other Options Are Wrong:
“Increases” contradicts data; “remains unchanged” ignores molecular interactions; “increases linearly” is not supported; “no definite trend” is false for similar chemical families.

Common Pitfalls:
Assuming a universal rule across dissimilar chemistries; the trend holds best for closely related liquids.

Final Answer:
decreases