9702 · 15.2
Equation of state — FAQ
Frequently asked questions for 9702 Equation of state. Direct answers first, then deeper explanation — then practise with marking.
Why do real gases behave ideally at high temperatures and low pressures?
At high temperatures, molecules have high kinetic energy, making the potential energy from intermolecular forces insignificant in comparison. At low pressures, molecules are far apart, so their individual volume is negligible compared to the container's volume. These conditions make the ideal gas assumptions more valid.
What is the difference between the molar gas constant (R) and the Boltzmann constant (k)?
The molar gas constant (R) is used in the macroscopic equation (PV=nRT) and relates to energy on a 'per mole' basis. The Boltzmann constant (k) is used in the microscopic equation (PV=NkT) and relates to energy on a 'per molecule' basis. They are connected by Avogadro's constant: R = k × N_A.
Can I use Celsius for calculations if I'm just looking at a change in temperature?
No, absolutely not. The gas laws are based on proportionality to absolute temperature (Kelvin), which has a true zero point. A change from 10°C to 20°C is not a doubling of temperature in the context of gas laws. You must always convert all temperatures to Kelvin before using them in any gas law formula.
When should I use the Combined Gas Law versus the Ideal Gas Law?
Use the Combined Gas Law (P₁V₁/T₁ = P₂V₂/T₂) when you have a fixed amount of gas changing from an initial state (1) to a final state (2). Use the Ideal Gas Law (PV=nRT or PV=NkT) when you need to find a property (P, V, n, T, N) of a gas in a single, specific state, or if the amount of gas changes.