Equilibrium Constant Calculator

About Equilibrium Constant Calculator

The equilibrium constant Kc measures the ratio of products to reactants at equilibrium. K > 1 means products favored.

How to use this calculator

1. Enter the equilibrium concentrations for the substances in your reaction, or use pressure values if you are finding \(K_p\).
2. Make sure each concentration is at equilibrium, not at the start of the reaction.
3. Match each substance to its coefficient in the balanced equation so the exponents are correct.
4. Click calculate to get the equilibrium constant and see the result immediately.

The formula explained

For a reaction like \(aA + bB \rightleftharpoons cC + dD\), the equilibrium constant \(K_c\) is found by dividing the product concentrations raised to their coefficients by the reactant concentrations raised to their coefficients. This shows the balance between products and reactants at equilibrium.

• \(K_c\) = the equilibrium constant in terms of concentration
• \(K_p\) = the equilibrium constant in terms of pressure
• [A] = the equilibrium concentration of reactant A
• [B] = the equilibrium concentration of reactant B
• [C] = the equilibrium concentration of product C
• [D] = the equilibrium concentration of product D
• a = the coefficient of A in the balanced equation
• b = the coefficient of B in the balanced equation
• c = the coefficient of C in the balanced equation
• d = the coefficient of D in the balanced equation

Step by step method

1. Write the balanced chemical equation and identify the reactants and products.
2. Substitute the equilibrium concentrations or pressures into the correct expression, using each coefficient as an exponent.
3. Compute the numerator and denominator separately, then divide to find the equilibrium constant.

Worked example

Problem. For the reaction \(2NO_2 \rightleftharpoons N_2O_4\), the equilibrium concentrations are \( [NO_2] = 0.40\\,M \) and \( [N_2O_4] = 1.6\\,M \). Find \(K_c\).

1. Use the formula \(K_c = \frac{[N_2O_4]}{[NO_2]^2}\).
2. Substitute the values, \(K_c = \frac{1.6}{(0.40)^2}\).
3. Calculate, \(K_c = \frac{1.6}{0.16} = 10\).

Tips and common mistakes

• Only use equilibrium values, because starting values do not give the equilibrium constant.
• Leave out pure solids and pure liquids from the expression, because their concentrations do not change in the same way as gases and dissolved substances.

Frequently asked questions