Kp ↔ Kc Conversion Calculator
Convert between the pressure and concentration equilibrium constants for a gas reaction — Kp = Kc(RT)^Δn — with Δn from the balanced equation and the constant in atm or bar.
How it works
Kp = Kc · (R·T)^Δn, Δn = Σ moles of gaseous products − Σ moles of gaseous reactants; R = 0.082057 L·atm/(mol·K) or 0.083145 L·bar/(mol·K)
Kc is written in molar concentrations and Kp in partial pressures; for ideal gases P = cRT links the two, so each gas-phase mole on the product side contributes a factor RT and each reactant mole divides by it. Only gases count — pure solids and liquids appear in neither constant. When gas moles are conserved (Δn = 0, as in H₂ + I₂ ⇌ 2HI) the constants are identical. Match R to the pressure unit your Kp uses.
Worked example
N₂ + 3H₂ ⇌ 2NH₃ at 400 K with Kc = 0.5: Δn = 2 − 4 = −2, RT = 32.8 L·atm/mol, Kp = 0.5 ÷ 32.8² = 4.6 × 10⁻⁴.
Frequently asked questions
Do units of K matter?
Strictly K is dimensionless (activities relative to 1 M or 1 bar); in practice Kp changes numerically with the pressure unit whenever Δn ≠ 0.
What about Kx (mole fraction)?
Kx = Kp · P^(−Δn) — it depends on total pressure, which is why Kp is preferred.
Related calculators
For study and planning — check units and conditions against your data source before using a result in the lab.