If a product has a low boiling point, what can be inferred about its vapor pressure?

• A. High
• B. Unknown
• C. Low
• D. Moderate

Answer: (A)

Vapor pressure is the pressure the vapor would exert in equilibrium with the liquid at a given temperature. Boiling occurs when this vapor pressure equals the ambient pressure. If a product has a low boiling point, it only needs a small temperature rise for its vapor pressure to reach that ambient pressure, so its vapor pressure at typical temperatures is relatively high. That’s why the correct inference is that the vapor pressure is high. If boiling point were higher, the vapor pressure at the same temperature would be lower. The option of unknown isn’t appropriate because the relationship is determined by the physical properties involved.