At a chosen temperature, use the lecture’s ideal gaseous mixture with p°=1 bar and positive partial pressures. Form Q from the current p_i/p° ratios, raising each to its stoichiometric power and dividing product factors by reactant factors. Q describes the current state; equilibrium gives Q_eq=K_p. The reaction Gibbs energy satisfies Δ_rG=Δ_rG°+RT ln Q, while Δ_rG°=-RT ln K_p. This pressure normalization makes both Q and K_p dimensionless.

Sources:

• https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Thermodynamics_and_Chemical_Equilibrium_%28Ellgen%29/15%3A_Chemical_Potential_Fugacity_Activity_and_Equilibrium/15.07%3A_Chemical_Potential_Activity_and_Equilibrium
• https://ocw.mit.edu/courses/5-60-thermodynamics-kinetics-spring-2008/52946f97fcb18850c32e9d4b8f6bab6a_5_60_lecture15.pdf