Convert between Kc and Kp for ideal gases
A Kp calculator converts a concentration-form equilibrium constant to a pressure-form constant, or performs the reverse conversion. Enter the known positive constant, the reaction temperature, and Δn for gaseous species. The page converts Celsius to kelvin when needed and shows the RT factor before applying Kp = Kc(RT)^Δn, making the convention visible instead of hiding it inside a unit selector.
This relationship is used for gas-phase equilibrium exercises under an ideal-gas approximation. It does not calculate K from a composition or determine Δn from an unbalanced equation. Count gaseous stoichiometric coefficients on the product side and subtract those on the reactant side. Do not include aqueous species, solids, or liquids in Δn even when they appear in the same balanced reaction.
Understand Δn and the RT exponent
For aA(g) + bB(g) ⇌ cC(g) + dD(g), Δn = (c + d) − (a + b). A positive Δn multiplies Kc by a positive power of RT, a negative Δn divides by that power, and Δn = 0 makes Kp numerically equal to Kc in this classroom convention. The reverse conversion divides by the same factor rather than changing the sign by guesswork.
The implemented gas constant is 0.082057366 L·atm·mol−1·K−1, paired with the familiar pressure/concentration conversion. Temperature must be positive in kelvins. Because modern thermodynamic K values are dimensionless activity ratios, the numerical conversion also assumes compatible standard states; copying a dimensional empirical constant from a different convention can make the equation meaningless even when the arithmetic runs.
Follow a Kp conversion example
For N2(g) + 3H2(g) ⇌ 2NH3(g), Δn = 2 − 4 = −2. If Kc = 0.50 at 450 K, then RT is about 36.9258 L·atm/mol and Kp = 0.50 × (36.9258)^−2, approximately 0.0003669. The negative exponent is the key reason Kp is smaller than Kc in this particular convention and temperature.
A reverse check starts with the displayed Kp and divides by (RT)^−2, which is equivalent to multiplying by (RT)^2. It should reproduce the original Kc within display rounding. If Δn is zero, both conversion directions should return the same number. If a result changes unexpectedly, recount gaseous coefficients and confirm that Celsius was not used directly as an absolute temperature.
Check reaction direction and standard states separately
Kp and Kc describe the same equilibrium but use different activity approximations. Neither value alone predicts reaction rate. A pressure reaction quotient must be compared with Kp, while a concentration reaction quotient must be compared with Kc under matching conditions. Mixing Qp with Kc can create a false direction conclusion even if each number was calculated correctly on its own basis.
Changing temperature generally changes the equilibrium constant itself, not only the algebraic RT conversion. The page converts K forms at the temperature associated with the known constant; it does not predict how K changes between temperatures. That separate problem requires thermodynamic data such as reaction enthalpy and a justified van ’t Hoff treatment over the interval.
Ideal-gas limits, units, and privacy
Real gases can require fugacity coefficients, and condensed phases or solutions require appropriate activities. Standard-state definitions, pressure units, concentration units, and the balanced equation must remain attached to a reported constant. Use a thermodynamics text, validated property package, or primary data source for research, process design, high-pressure systems, or any decision where an ideal-gas classroom conversion is insufficient.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.