When do I use Kc vs Kp?

Use Kc for equilibria expressed in molar concentrations (gases and aqueous solutes). Use Kp for gaseous equilibria expressed in partial pressures (atm).

Which phases are excluded from K?

For Kc, pure solids and pure liquids are omitted. For Kp, only gases appear; solids, liquids, and aqueous species are excluded.

How do I convert between Kc and Kp?

Use Kp = Kc(RT)^Δn, where Δn is moles of gaseous products minus moles of gaseous reactants, R = 0.082057 L·atm·mol⁻¹·K⁻¹, and T is in Kelvin.

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Equilibrium Constant Calculator (K_c & K_p)

Build an equilibrium expression from species (role, stoichiometric ν, phase) and compute K_c (using concentrations) or K_p (using partial pressures). Solids and liquids are excluded automatically from K_c; only gases count for K_p. Optionally convert between K_c and K_p via Δn and T.

Background

For a reaction aA + bB ⇌ cC + dD, K = (activities of products)^coeff / (activities of reactants)^coeff. We approximate activities with concentrations for K_c and with partial pressures for K_p. Conversion uses K_p = K_c(RT)^Δn with Δn = (sum ν gases products − sum ν gases reactants).

Choose mode, add species, and enter values:

Equilibrium mode:

K_c (use concentrations, mol/L)

K_p (use partial pressures, atm)

Temperature (optional):

Units: K °C Provide T to enable K_c ⇄ K_p via K_p = K_c(RT)^Δn.

Species & values:

Enter each species with: role, stoichiometric coefficient ν, phase, and its concentration (mol/L) for K_c or partial pressure (atm) for K_p.

Name (optional)	Role	ν	Phase	Value (mol/L)	Include

Quick picks:

Result:

No results yet. Choose mode, add species, and enter values.

How to use this calculator

K_c mode: enter concentrations (mol/L) for gases/aqueous. Solids/liquids are auto-excluded.
K_p mode: enter partial pressures (atm) for gases only. Non-gases are auto-excluded.
Optionally provide T to convert K_c ⇄ K_p with Δn (gases).

Formula & Equations Used

K_c = \( \dfrac{\prod [\mathrm{product}]^{\nu}}{\prod [\mathrm{reactant}]^{\nu}} \) using gas + aqueous species
K_p = \( \dfrac{\prod (P_{\mathrm{product}})^{\nu}}{\prod (P_{\mathrm{reactant}})^{\nu}} \) using gaseous species
Δn = Σν(products, gas) − Σν(reactants, gas)
K_p = K_c(RT)^Δn, R = 0.082057 L·atm·mol⁻¹·K⁻¹

Examples

Example 1 (K_p, heterogeneous)

CaCO₃(s) ⇌ CaO(s) + CO₂(g). Only the gas appears in K_p: K_p = P(CO₂).

Example 2 (K_c, aqueous)

Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺. K_c = [FeSCN²⁺]/([Fe³⁺][SCN⁻]).

Frequently Asked Questions

Q: Why do solids & liquids drop out?

Their activity is ~1, so they don’t appear in K expressions.

Q: What if one side ends up empty?

That side becomes 1. This often happens in heterogeneous equilibria.

Q: Which units for K_p?

Use atm so R = 0.082057 and the (RT)^Δn conversion is consistent.

Intro to Chemical Equilibrium

Equilibrium Constant Calculator (Kc & Kp)