Vapor pressure is usually expressed in which units?

• A. Pascals (Pa)
• B. Atmospheres (atm)
• C. Millimeters of mercury (mmHg)
• D. Pounds per square inch (psi)

Answer: (C)

Vapor pressure is a pressure, and historically scientists measured pressures with a mercury manometer. The height of the mercury column directly represents the pressure, so vapor pressure data have long been reported in millimeters of mercury. Since 1 atm equals 760 mmHg, this unit provides an immediate sense of how the vapor pressure compares to atmospheric pressure, and many standard tables use mmHg (often referred to as Torr). Modern practice also uses pascals or kilopascals, but for traditional chemical data and vapor-pressure correlations, millimeters of mercury is the common expression.