Ideal Gas Law Calculator
How It Works
The ideal gas law PV = nRT relates the four state variables of an ideal gas: pressure (P), volume (V), amount in moles (n), and temperature (T). R is the universal gas constant, equal to 0.08206 L·atm/(mol·K).
This calculator solves for any one unknown variable when the other three are provided. Set the unknown to 0 (or -999 for temperature) and the calculator will determine its value. Temperature must be in Kelvin for the equation (the calculator converts from Celsius automatically).
At STP (Standard Temperature and Pressure: 0°C and 1 atm), one mole of an ideal gas occupies exactly 22.4 liters. Real gases deviate from ideal behavior at high pressures and low temperatures, where intermolecular forces become significant. The van der Waals equation accounts for these deviations.
Frequently Asked Questions
What are the units for the ideal gas law?
When using R = 0.08206 L·atm/(mol·K): P in atmospheres, V in liters, n in moles, T in Kelvin. Other R values exist for different unit combinations (e.g., R = 8.314 J/(mol·K) for SI units).
What is STP in chemistry?
STP (Standard Temperature and Pressure) is 0°C (273.15 K) and 1 atm. At STP, one mole of an ideal gas occupies 22.414 liters. IUPAC updated STP to 0°C and 1 bar in 1982, giving a molar volume of 22.711 L.
When does the ideal gas law fail?
The ideal gas law assumes no intermolecular forces and zero molecular volume. It fails at high pressures (above ~10 atm), low temperatures (near condensation point), and for polar molecules. Use the van der Waals equation for more accuracy.
How do I convert between pressure units?
1 atm = 101.325 kPa = 760 mmHg = 760 torr = 14.696 psi = 1.01325 bar. Make sure to use consistent units with the appropriate R constant.