Chemistry · Gases
Ideal gas law calculator.
Pick the unknown, enter the other three, and PV = nRT is solved with the units converted for you.
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The ideal gas law
PV = nRT, where P is pressure, V is volume, n is moles of gas, T is absolute temperature in kelvin, and R is the molar gas constant, 8.314 462 618 J mol-1 K-1 (the CODATA value from NIST). The tool converts everything to SI units (pascal, cubic metre, kelvin), solves, and converts back. Kelvin is C + 273.15. Unit factors: 1 atm = 101 325 Pa, 1 bar = 100 000 Pa, 1 mmHg = 133.322 Pa and 1 psi = 6894.757 Pa (NIST SP 811).
Worked example 1. Volume of 1.00 mol at 273.15 K and 101.325 kPa: V = nRT / P = 1 x 8.314462618 x 273.15 / 101 325 = 22.414 L. Worked example 2. Moles of gas in 10 L at 1 atm and 25 C (298.15 K): n = PV / RT = 0.4087 mol.
The ideal gas law is a model. It works best for gases at low pressure and high temperature, and it is less accurate for real gases near condensation or at high pressure. For pressurized cylinders and safety-critical work, use manufacturer data and qualified guidance.
Frequently asked questions
What is PV = nRT?
The ideal gas law: pressure times volume equals moles times the gas constant times absolute temperature.
Which value of R should I use?
This tool uses 8.314462618 J/(mol K), which equals 0.0820574 L atm/(mol K) when pressure is in atm and volume in litres. The units just have to agree, and the tool does the conversion.
Why must temperature be in kelvin?
The law uses absolute temperature, where zero means no thermal motion. Celsius and Fahrenheit values are converted to kelvin first.
When is the ideal gas law inaccurate?
At high pressure or low temperature, where molecules attract each other and take up real volume. Use a real-gas equation or tabulated data then.
Sources
Checked
- OpenStax Chemistry 2e, 9.2 The Ideal Gas Law
- NIST CODATA: molar gas constant
- NIST SP 811, Appendix B.8: Factors for Units Listed Alphabetically (atm, bar, mmHg, psi)
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.