The formula
P · V = n · R · T
| Symbol | Quantity | Unit |
|---|---|---|
| P | Pressure | Pa |
| V | Volume | m³ |
| n | Moles | mol |
| T | Temperature | K |
What it means
The ideal gas law treats gas particles as points with no volume and no attraction between them. Real gases depart from it, but under ordinary conditions the approximation is remarkably good.
How to work it out by hand
- Convert the temperature to kelvin: always kelvin, never Celsius
- Use consistent units for pressure and volume
- Rearrange for whichever quantity you need
What is worth knowing
Using Celsius instead of kelvin is the classic error, and it is not a small one: the law is proportional to absolute temperature, so 0 °C would predict zero pressure. Add 273.15 first, every time. The law fails where its assumptions do — at high pressure, where particle volume stops being negligible, and at low temperature, where attractions matter and the gas is close to condensing. Van der Waals' equation patches both, at the cost of two extra constants per gas.