The ideal gas law combines the relationships outlined in Boyle’s law, Charles’s law, and Avogadro’s
law to yield an expression that can be used to predict the behavior of a gas. The ideal gas law shows
the relationship between four variables that define a sample of gas—pressure (P), volume (V),
temperature (T), and number of moles (n)—and is represented by the equation
PV = nRTBASIC CONCEPT
All the information in Boyle’s law, Charles’s law, and Avogadro’s law is contained in the ideal
gas equation:PV = nRTThe constant R is known as the (universal) gas constant. Under STP conditions (273.15 K and 1
atmosphere), 1 mole of gas was shown to have a volume of 22.4 L. Substituting these values into the
ideal gas equation gave R = 8.21 × 10–2 L • atm/(mol • K).
The gas constant may be expressed in many other units; another common value is 8.314 J/(K • mol),
which is derived when the SI units of pascals (for pressure) and cubic meters (for volume) are
substituted into the ideal gas law. When carrying out calculations based on the ideal gas law, it is
important to choose a value of R that matches the units of the variables.
Example: What volume would 12 g of helium occupy at 20 ̊C and a pressure of 380 mm Hg?
Solution: The ideal gas law can be used, but first, all of the variables must be converted to yield
units that will correspond to the expression of the gas constant as 0.0821 L • atm/(mol •
K).