Equation of state — common mistakes

Always remember to convert temperatures into Kelvin (K) for all gas law calculations. Failing to do so is a very common mistake in exams!

Pay close attention to units! Ensure pressure is in Pascals, volume in cubic metres, and temperature in Kelvin. Incorrect units are a common source of error in calculations.

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.

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.

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.

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.