Converting Between Kp and Kc

Converting between Kp and Kc means computing Δn from the balanced equation's gas-phase coefficients alone, then substituting Δn, temperature in kelvin, and the gas constant R into Kp = Kc(RT)^Δn, solving for whichever constant is unknown. Any species that is not a gas is left out of the Δn count entirely, the same omission rule heterogeneous equilibria already follow.

For 2 SO2 + O2 ⇌ 2 SO3 at 727 °C, first converted to 1000 K, Δn = 2 − 3 = −1. Given Kc = 4.32, Kp = Kc(RT)^(−1) = 4.32/[(0.0821)(1000)], about 0.0526, a smaller number than Kc since Δn is negative here.