5.1 Pressure

• A gas uniformly fills any container, is easily compressed, and mixes completely with any other gas.
• One of the most obvious properties of a gas is that it exerts pressure on its surroundings.
• A device to measure atmospheric pressure, the barometer, was invented in 1643 by an Italian scientist named Evangelista Torricelli (1608-1647), who had been a student of Galileo.

Units of Pressure

• Because instruments used for measuring pressure, such as the manometer, often contain mercury, the most commonly used units for pressure are based on the height of the mercury column (in millimeters) that the gas pressure can support.
• The unit mm Hg (millimeter of mercury) is often called the torr in honor or Torricelli.
  • The terms torr and mm Hg are used interchangeably by chemists.
• A related unit for pressure is the standard atmosphere (abbreviated atm):
• $\ce{1 standard atmosphere = 1 atm = 760 mm Hg = 760 torr}$
• Since pressure is defined as force per unit area, $\ce{Pressure = \frac{force}{area}}$ the fundamental units of pressure involve units of force divided by units of area.
• The unit of pressure in the SI system is newtons per meter squared ($\ce{N/m^2}$) and is called the pascal (Pa).
  • In terms of pascals, the standard atmosphere is $\ce{1 standard atmosphere = 101,325 Pa}$.

The pressure of a gas is measured as 9 torr. Represent this pressure in bot atmospheres and pascals. $\ce{49 torr \times \frac{1 atm}{760 torr} = 6.4 \times 10^{-2} atm}$ $\ce{6.4 \times 10^{-2} atm \times \frac{101,325 Pa}{1 atm} = 6.5 \times 10^3 Pa}$

5.2 The Gas Laws of Boyle, Charles, and Avogadro

Boyle’s Law

• Boyle’s law: The product of the pressure and volume for the trapped air sample is constant within the accuracies of Boyle’s measurements.
  • $PV = k$
  • k is a constant for a given sample of air at a specific temperature.
• There is an inverse relationship between pressure and volume.
• A gas that strictly obeys Boyle’s law is called an ideal gas.