Calculate Kc from an equilibrium composition
An equilibrium constant calculator combines equilibrium concentrations with the stoichiometric coefficients from a balanced reaction. Enter one row for each species with its name, side, coefficient, phase, and equilibrium concentration in mol/L. The page builds the concentration quotient with product terms in the numerator and reactant terms in the denominator, then shows the logarithmic contributions behind the final Kc value.
Use values measured or supplied after the system has reached the same equilibrium state. Initial concentrations belong in an ICE-table calculation, not directly in this final expression unless they happen to equal the equilibrium values. The calculator does not solve an unknown equilibrium composition, balance the reaction, or infer a coefficient from a formula; those pieces must come from the problem before Kc is evaluated.
Build the Kc expression from coefficients and phases
For aA + bB ⇌ cC + dD, the classroom concentration expression is Kc = [C]^c[D]^d ÷ ([A]^a[B]^b). Each stoichiometric coefficient becomes an exponent. The page adds logarithms of product terms and subtracts logarithms of reactant terms, then exponentiates once. This is numerically safer than forming several very large or very small powers independently.
Pure solids and pure liquids have effectively constant activity in this model and are omitted only when you explicitly mark their phase as s or l. Aqueous and gaseous entries remain in the concentration expression. Every included value must be positive because zero has no finite logarithm and an equilibrium constant is not obtained by dividing by a zero reactant concentration.
Follow an equilibrium constant example
For N2(g) + 3H2(g) ⇌ 2NH3(g), suppose the equilibrium concentrations are 0.20, 0.60, and 0.40 mol/L. The expression is Kc = [NH3]^2 ÷ ([N2][H2]^3). Substitution gives 0.40^2 ÷ (0.20 × 0.60^3), approximately 3.7037 under the stated concentration convention.
If a reaction includes CaCO3(s) ⇌ CaO(s) + CO2(g), the two pure solid terms are omitted and the remaining expression uses only the carbon dioxide activity or its classroom concentration approximation. Do not omit a dissolved species merely because its concentration stays nearly constant in one experiment; phase and standard-state reasoning, not convenience, determines whether a term belongs in the expression.
Interpret Kc without overreading its size
A large Kc indicates that products are favored at equilibrium under the stated temperature and standard-state convention, while a small value favors reactants. It does not say how quickly equilibrium is reached. A reaction can have a large equilibrium constant and still proceed slowly because kinetics and activation barriers answer a different question. Temperature must accompany any meaningful equilibrium constant comparison.
Reverse the balanced reaction and the new equilibrium constant is 1/Kc. Multiply every balanced coefficient by a factor and the new constant is Kc raised to that factor. These checks help catch missing exponents. Numerical precision should follow the concentration data and model, not every digit shown by the calculator, especially when concentrations are approximate or activities differ from concentrations.
Activity limits, experimental meaning, and privacy
Thermodynamic equilibrium constants are dimensionless because activities are ratios to standard states. Direct concentration powers are a common teaching approximation and can be inadequate for concentrated electrolytes, gases far from ideality, mixed solvents, or systems with complex formation. Use activity coefficients, fugacity, temperature-specific data, and a suitable equilibrium model when accuracy or consequential decisions require them.
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.