Gas Law Calculator
Ideal gas law, combined gas law, Graham's law of effusion, and the van der Waals real-gas correction. Pick a mode and leave the unknown blank to solve for it.
How to use this tool
Solve the common gas-law relationships between pressure, volume, temperature and amount. Pick the law you need and leave the one unknown blank, the tool fills it in.
What to enter
- Mode: Ideal gas (PV = nRT), Combined (one state → another), Graham's (effusion rates from molar mass), or van der Waals (real-gas pressure correction).
- The values: fill in all but one. Use absolute units: pressure in atm, volume in litres, temperature in kelvin (K = °C + 273.15), amount in moles.
Reading the result
The blank quantity is solved and shown. In van der Waals mode you also see the ideal-gas pressure alongside the corrected one, so you can judge how far the gas departs from ideal behaviour (largest at high pressure and low temperature).
Worked example
Ideal mode: enter n = 1 mol, T = 273.15 K, P = 1 atm and leave V blank, it solves V ≈ 22.41 L, the molar volume of a gas at STP.
Inputs
Result
The quantity you left blank is solved and shown. In van der Waals mode the ideal pressure sits alongside the corrected one, the deviation grows at high pressure and low temperature, where real gases stray furthest from ideal.
Methodology
Ideal gas: PV = nRT with R = 0.0820573 L·atm·mol⁻¹·K⁻¹. Enter any three of P, V, n, T and the fourth is solved.
Combined: P₁V₁/T₁ = P₂V₂/T₂. Enter five of the six values; leave the unknown blank.
Graham: rate₁/rate₂ = √(M₂/M₁) for effusion of two gases by molar mass.
van der Waals: (P + a·n²/V²)(V − n·b) = nRT, solved for P, with the ideal-gas pressure shown for comparison. Constants a, b from the CRC Handbook.
Known limits
- Temperatures are absolute (K); pressures in atm, volumes in litres. Convert before entering.
- The ideal law is least accurate at high pressure and low temperature, where the van der Waals mode is more realistic.