Kp from Kc (Kp = Kc(RT)^Δn)

Gas-phase equilibria can be written with partial pressures or with molar concentrations, and the ideal gas law (P = (n/V)RT) converts between them one species at a time. Every mole of gas on the product side contributes a factor of RT and every mole on the reactant side removes one, so only the net change Δn survives: Kp = Kc$(RT)^{\Delta n}$. This calculator uses R = 0.08206 L·atm/(mol·K), the value that pairs partial pressures in atmospheres with concentrations in mol/L.

Δn counts gases only: solids and liquids never appear in either constant. For the Haber synthesis N₂ + 3H₂ ⇌ 2NH₃, Δn = 2 − 4 = −2, so Kp is far smaller than Kc at high temperature. Where Δn = 0, as in H₂ + I₂ ⇌ 2HI, the two constants are numerically identical and the whole conversion evaporates. A worked case: a reaction with Δn = +1 and Kc = 1.00 at 1000 K has Kp = 1.00 × (0.08206 × 1000) = 82.1, a reminder that "the" equilibrium constant is meaningless until you say which basis you meant.