The ideal gas law combines Boyle’s, Charles’s and Avogadro’s laws into a single relationship between pressure, volume, temperature and quantity.
Temperature must be absolute. Using Celsius here is the most common error in the entire topic and it produces answers that are wrong by hundreds of percent. Convert to kelvin by adding 273.15 before you do anything else.
Matching your units to R
R takes different numerical values depending on the units. Use 8.314 with pascals, cubic metres and kelvin. Use 0.08206 with atmospheres and litres. Mixing the two is the second most common error.
Molar volume
At standard temperature and pressure — 0 °C and 1 atm — one mole of any ideal gas occupies 22.4 litres, regardless of what gas it is. A mole of hydrogen and a mole of carbon dioxide fill the same space, because the law contains no term for molecular size or mass. At 25 °C the figure is 24.8 litres.
When gases stop being ideal
The model assumes molecules have no volume and do not attract each other. Both assumptions fail at high pressure and low temperature, which is exactly where gases liquefy. The van der Waals equation adds correction terms for both effects and is the usual next step.