Equilibrium Constant - Kc, Kp and Delta n

An equilibrium constant calculator is a tool that applies the law of mass action to a balanced equation of the form aA + bB reversible arrow cC + dD. You enter the four stoichiometric coefficients and the four equilibrium concentrations (for Kc) or partial pressures (for Kp), and the calculator returns K, the change in moles of gas delta n, and a Kp to Kc conversion when the temperature is known.

• • General chemistry homework: Check Kc and Kp values for textbook problems on esterification, the Haber process, or weak acid dissociation.
• • AP and Olympiad practice: Compute K for a balanced equation and compare to a published value to judge the direction of shift.
• • Lab prep for reversible syntheses: Estimate K for esterification or amide hydrolysis before scaling the reaction.
• • Buffer and speciation work: Evaluate the equilibrium expression for acid dissociation constants when pH and total concentration are known.

The equilibrium constant summarizes a reversible reaction in a single dimensionless ratio. The calculator evaluates that ratio for any balanced equation with up to two reactants and two products, which covers most reactions a student meets.

Before you plug numbers into the equilibrium expression, run the reaction through the Chemical Equation Balancer Calculator so the stoichiometric coefficients reflect the actual balanced equation.

The calculator reads the mode (Kc or Kp), the four stoichiometric coefficients, and the four equilibrium concentrations or pressures, then evaluates the law of mass action. For Kp mode it also reads the temperature in kelvin so it can recover the matching Kc.

• a, b, c, d: Stoichiometric coefficients of A, B, C, and D from the balanced equation. A coefficient of 0 omits that species, the standard treatment for pure solvents such as water.
• [A], [B], [C], [D]: Equilibrium concentrations in mol/L (Kc mode) or equilibrium partial pressures in atm (Kp mode). Each value must be positive.
• T (kelvin): Absolute temperature used only in Kp mode to convert Kp to Kc via Kp = Kc (R T)^delta n with R = 0.08206 L atm / (mol K).
• delta n: Change in moles of gas: (c + d) - (a + b). Positive delta n means more gas on the product side; negative means more gas on the reactant side.

When any stoichiometric coefficient is zero the calculator omits that term from the equilibrium expression, which is the standard treatment for pure solids and solvents such as water. The same expression also defines the reaction quotient Q, so the calculator returns Q at any set of compositions; the result is K at equilibrium and Q otherwise.

According to IUPAC Gold Book, the equilibrium constant K is the product of product activities raised to stoichiometric coefficients divided by the product of reactant activities raised to stoichiometric coefficients.

When you also need to figure out how much of each reactant is consumed at equilibrium, Stoichiometry Reaction Calculator sets up the limiting-reactant side of the calculation.

Four ideas explain every number the equilibrium constant calculator returns, from the law of mass action to the limits of the concentration-based form.

These four ideas cover every result the equilibrium constant calculator returns, and they explain why the same reaction can give a different K depending on whether you measured concentrations, partial pressures, or activities.

K connects to kinetics through the activation energy barrier, which Activation Energy Calculator estimates from Arrhenius plots when you have rate constants at two temperatures.

Enter the balanced equation and the equilibrium concentrations (or pressures); the calculator returns the equilibrium constant together with delta n and the matching Kc in Kp mode.

1. 1 Pick the mode: Select Kc for molar concentrations in mol/L (most aqueous chemistry), or Kp for partial pressures in atm (most gas-phase chemistry).
2. 2 Enter the stoichiometric coefficients: Type a, b, c, and d from the balanced equation. Use 0 if a species does not appear.
3. 3 Enter the temperature for Kp mode: Set the absolute temperature in kelvin; used only to convert Kp to Kc. Use 298.15 K for 25 C reference data.
4. 4 Enter the equilibrium concentrations: Type the equilibrium molarity of each species (Kc) or partial pressure in atm (Kp). All four values must be positive.
5. 5 Read the equilibrium constant: The calculator reports K together with delta n. In Kp mode it also reports the recovered Kc value.
6. 6 Check delta n for the Kp to Kc conversion: When delta n equals zero Kp and Kc are equal. When delta n is positive Kp is larger than Kc; when delta n is negative Kp is smaller.

For acetic acid dissociation at 25 C with [CH3COOH] = 0.10 mol/L, [CH3COO-] = 0.00132 mol/L, and [H+] = 0.00132 mol/L, the calculator returns Kc = 1.74e-5 and delta n = 1. Kp is not the natural form for an aqueous dissociation because Kp describes gas-phase partial pressures.

To turn the molarity you measured back into a mass to weigh on the bench, Grams to Moles Calculator converts grams and molar mass into moles before you recompute K.

An equilibrium constant calculator turns the qualitative phrase 'position of equilibrium' into a number you can plan a reaction around.

• • Direct K from concentrations: Skip the algebra: enter the four concentrations and stoichiometric coefficients and read the equilibrium constant.
• • Kc and Kp in one tool: Switch modes without re-entering the reaction; the Kp mode converts back to Kc using (R T)^delta n.
• • Verified worked examples: Acetic acid dissociation and the Haber process reference cases match the values textbooks publish.
• • Works at non-equilibrium compositions: The same expression evaluates Q at any concentration, so the calculator works for equilibrium and non-equilibrium compositions.
• • Compact output panel: K, delta n, and Kc (in Kp mode) are visible side by side so you can compare with textbook values at a glance.

If you also need to plan how much of each reactant to weigh out before the reaction, pair the equilibrium constant calculator with the grams-to-moles tool to convert molarity into a bench-scale mass.

Four variables drive the equilibrium constant the calculator returns, and they explain why textbook K values depend on temperature, ionic strength, and the form (Kc or Kp) you measure.

• • The equilibrium expression assumes activities equal concentrations. In concentrated solutions the actual K (in terms of activities) differs from the value the calculator returns.
• • Only two reactants and two products are supported. For reactions with three or more species on one side, run the equation through a chemical equation balancer first.
• • Temperature is fixed for each calculation. To study how K changes with temperature, use the Arrhenius or van 't Hoff equation alongside this calculator.

These four factors are why the same textbook reaction can return slightly different K values in different reference tables. The calculator reports the analytical-concentration K used in most general-chemistry and AP-Chemistry problems.

According to NIST CODATA, the molar gas constant R equals 8.314462618 J mol^-1 K^-1 in SI units and 0.082057 L atm mol^-1 K^-1 in atm units.

According to Chemistry LibreTexts, when Q is less than K the reaction proceeds forward, when Q equals K the system is at equilibrium, and when Q is greater than K the reaction proceeds in reverse.

When you need to study how K changes with temperature, Arrhenius Equation Calculator fits rate constants to the Arrhenius form so the temperature dependence is reproducible.