Ideal Gas Law Calculator

In short PV = nRT relates the four things you can measure about a gas, and R is 8.314462618 joules per mole per kelvin — exact since 2019, when the kelvin was redefined from the Boltzmann constant. Give any three and the fourth follows. Temperature must be in kelvin, because the law needs a scale where zero means zero: at −273.15 °C an ideal gas would have no volume at all, and below it the arithmetic returns a negative one. The most quoted consequence is the molar volume, and this is where two right answers exist. One mole of an ideal gas at 0 °C and 100 kPa occupies 22.711 litres, which has been the IUPAC standard since 1982. At 0 °C and 1 atmosphere — the older definition — it occupies 22.414 litres, and that 22.4 is what most people were taught. The two differ by 1.32%, so a question about molar volume at STP has two defensible answers depending on which textbook you learned from, and it is worth saying which you mean. At 25 °C and 1 atm, closer to an actual bench, a mole fills 24.465 litres. The law is called ideal because it assumes molecules take up no space and do not attract one another. Real gases do both, so near room conditions the law is typically good to about 1%, and near condensation or at high pressure it drifts by tens of per cent. That is the point at which the van der Waals equation or a compressibility factor is worth reaching for.