How it works
The ideal gas law relates the pressure, volume, amount and absolute temperature of a gas. It uses the molar gas constant R = 8.314462618 J/(mol·K), which is exact in the 2019 SI.
Temperature must be absolute (kelvin). With pressure in kPa and volume in liters, R has the same numerical value because 1 kPa·L = 1 J.
Real gases follow this law closely at ordinary temperatures and pressures. It becomes less accurate at high pressure or near the point where the gas would condense.
Formula
P × V = n × R × T, R = 8.314462618 J/(mol·K)
Questions people ask
What volume does one mole of gas occupy?
At 0 °C and 1 atm (101.325 kPa), 22.414 L. At 0 °C and 100 kPa (current IUPAC STP), 22.711 L. At 25 °C and 1 atm, about 24.47 L.
Why must temperature be in kelvin?
The law is proportional to absolute temperature. Using °C would give zero or negative volumes at temperatures where real gases clearly still have volume.
What are the other values of R?
0.082057 L·atm/(mol·K), 8.314462618 kPa·L/(mol·K) and 62.3636 L·Torr/(mol·K). They are the same constant in different units.