PV = nRT
The ideal gas equation links pressure P, volume V, amount n and temperature T. The gas constant R depends on the units:
| Value of R | Use with |
|---|---|
| 0.0821 L atm K⁻¹ mol⁻¹ | P in atm, V in litres |
| 8.314 J K⁻¹ mol⁻¹ | P in Pa, V in m³ (SI) |
| 0.0831 L bar K⁻¹ mol⁻¹ | P in bar, V in litres |
| 1.987 cal K⁻¹ mol⁻¹ | energy in calories |
Always use kelvin. T(K) = T(°C) + 273.15. Using °C is the most common mistake in gas law questions.
Molar volume
One mole of an ideal gas takes up 22.4 L at 0 °C and 1 atm (the old STP), or 22.7 L at 0 °C and 1 bar (the current IUPAC STP). NCERT uses 22.7 L; many older books and papers use 22.4 L, so check which one the question means.
Need the moles from a mass? Use the molar mass calculator.
Questions students ask
What is the value of R in L atm?
0.0821 L atm K⁻¹ mol⁻¹. In SI units it's 8.314 J K⁻¹ mol⁻¹.
What volume does 1 mole of gas occupy at STP?
22.4 L at 273 K and 1 atm, or 22.7 L at 273 K and 1 bar (IUPAC's current STP, used by NCERT).
Does PV = nRT work for real gases?
Approximately, at low pressure and high temperature. At high pressure or low temperature, real gases deviate, and the van der Waals equation is used instead.