MCQ Detailed View

The term atmospheric pressure refers to the pressure exerted by the weight of the air column above the Earth’s surface. At sea level and at 0°C, the normal or standard atmospheric pressure has a well-defined value. This pressure can be expressed in different but equivalent units.

In the SI unit system, atmospheric pressure is often measured in atmospheres (atm). One standard atmosphere is defined as the average pressure at sea level. Its value is:

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  1 atm

  
  

This same pressure can also be expressed in terms of Torr. The unit Torr was named after Evangelista Torricelli, the inventor of the barometer. One atmosphere equals:

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  760 Torr

  
  

Another common unit is based on the height of a column of mercury supported by atmospheric pressure. At sea level, standard pressure is sufficient to support a mercury column of about 76 centimeters (or 760 millimeters). Therefore:

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  76 cm of Hg

  
  

Since all these values represent the same physical quantity — normal atmospheric pressure — the correct answer is “All of these.”

Understanding standard atmospheric pressure is important in physical chemistry because it provides the reference point for gas laws and thermodynamic calculations. Boyle’s Law, Charles’s Law, and the Ideal Gas Law all rely on this concept. Standard conditions, known as STP (Standard Temperature and Pressure), are based on a temperature of 0°C (273 K) and a pressure of 1 atm.

This definition is crucial in laboratory work, calibration of instruments, and comparison of experimental data. Whether expressed in atm, Torr, or cm of Hg, these values represent the same standard reference pressure at sea level and 0°C.