Gases exhibit remarkable uniformity in physical behaviour regardless of their chemical identity. The ideal gas is a theoretical model that allows generalised mathematical treatment.
The Four Postulates
You must be able to state these clearly in exam responses:
1. Gas particles are in constant, random, straight-line motion.
2. The particles themselves possess negligible volume compared to the container. The gas is effectively mostly empty space.
3. There are no intermolecular forces of attraction or repulsion between the particles.
4. All collisions are perfectly elastic. No net loss of kinetic energy.
Avogadro's Law and Molar Volume
📘 Avogadro's Law
Equal volumes of any gases, under identical conditions of temperature and pressure, contain an identical number of molecules (and therefore moles). The identity or mass of the gas is irrelevant.
Molar Volume at STP
IUPAC defines Standard Temperature and Pressure (STP) as 273.15 K and exactly 100 kPa.
At STP, one mole of any ideal gas occupies:
\(V_m = 22.7 \text{ dm}^3 \text{ mol}^{-1}\)
Quick moles calculation at STP: \( n = \frac{V}{22.7} \) (volume must be in dm³)