Equilibrium Constant Calculator

How to use this equilibrium constant calculator

What this calculator does

This calculator evaluates the concentration-based equilibrium constant, Kc, for a reversible reaction written as aA + bB ⇌ cC + dD. It raises each equilibrium concentration to its stoichiometric coefficient, multiplies the product-side terms, multiplies the reactant-side terms, and divides the two. The result describes the composition ratio at equilibrium for the stated temperature and reaction equation. It does not predict how fast equilibrium is reached, replace an equilibrium experiment, or determine a new equilibrium after conditions change.

When to use it

Use it to check a laboratory calculation from measured equilibrium concentrations, compare whether a reaction mixture is product- or reactant-favored, verify an equilibrium expression from a balanced equation, or prepare a reproducible spreadsheet for a report. The underlying idea is summarized in the Chemistry LibreTexts explanation of the equilibrium constant.

How to calculate

1. The calculator opens with a complete sulfur trioxide demonstration: a = 2, [A] = 0.03 mol/L, b = 1, [B] = 0.035 mol/L, c = 2, [C] = 0.5 mol/L, d = 0, and [D] = 1 mol/L. Its example workbook is immediately available.
2. Replace the coefficients with those from your balanced equation. Coefficients must be nonnegative; use 0 to omit the second reactant or product.
3. Enter each equilibrium concentration in mol/L using a period as the decimal separator. Scientific notation is accepted, but thousands separators and decimal commas are not.
4. Read Kc, the product and reactant terms, log₁₀(Kc), Δn, and the contribution table. Select Download Excel to export the current validated model.
5. Reset clears the demonstration data and all calculated content. Download Excel is then disabled until a complete valid state is entered again.

Input guide

Coefficient a, b, c, and d are required nonnegative plain numbers from 0 through 100. They are unitless and normally come from the balanced chemical equation. For example, a = 2 means [A][A], [B], [C], and [D] are equilibrium molar concentrations. Each field accepts a positive decimal or scientific-notation value up to 1e100 when its coefficient is greater than 0. A value such as 0.035 means 0.035 mol/L. A zero or negative concentration is invalid for an included species because logarithmic and power calculations would be undefined or physically unsuitable here. When a coefficient is 0, its concentration is ignored and may be left blank after editing.

Output guide

Equilibrium constant Kc is the primary ratio. Kc greater than 1 indicates that products are favored in the equilibrium composition; Kc below 1 indicates reactants are favored; Kc near 1 indicates neither side strongly dominates. Product term is [C]^c × [D]^d, while Reactant term is [A]^a × [B]^b. log₁₀(Kc) compresses very large and very small values onto a readable scale. Concentration exponent Δn equals c + d – a – b and helps show the formal concentration-power difference. The summary pills repeat model validity, favored side, and overall coefficient sum. The table lists each active species, coefficient, concentration, and powered contribution used in the calculation.

Worked example

For 2A + B ⇌ 2C with [A] = 0.03 mol/L, [B] = 0.035 mol/L, and [C][C]^c × [D]^d) / ([A]^a × [B]^b)

The exponents come from the balanced reaction, not from measured amounts. Pure solids and pure liquids are ordinarily omitted from equilibrium expressions because their activities are treated as constant; this calculator is intended for species represented by equilibrium concentrations. The IUPAC Gold Book definition of an equilibrium constant provides the formal thermodynamic context, while the reaction quotient overview from Chemistry LibreTexts explains the same expression before equilibrium is reached.

Interpretation, assumptions, and common mistakes

Kc is tied to a particular balanced equation and temperature. Reversing the reaction inverts Kc, and multiplying all reaction coefficients by a factor raises Kc to that factor. Concentration values must describe the equilibrium state; initial concentrations generally cannot be substituted unless the problem has already solved for equilibrium composition. A catalyst changes the speed at which equilibrium is approached but not the equilibrium constant itself. The Le Châtelier's principle discussion is useful for understanding how a system responds to changes while distinguishing those shifts from the temperature-dependent value of K.

Common mistakes include using unbalanced coefficients, mixing mol/L with another concentration unit, inserting initial rather than equilibrium concentrations, including a pure solid as though it were dissolved, or rounding intermediate powers too early. Keep full precision through the calculation and round only the displayed result. Extremely large or small values are shown in scientific notation to preserve readability.