Equilibrium Constant Calculator

Created by Dominika Śmiałek, MD, PhD candidate
Reviewed by Dominik Czernia, PhD candidate and Jack Bowater
Last updated: Jan 11, 2022

This equilibrium constant calculator will help you understand reversible chemical reactions, which are reactions in which both the forward and backward reaction are occurring simultaneously. After a certain amount of time an equilibrium is formed, meaning that the rate of reactants being turned into products is the same as the rate of products being turned back into reactants. At this point the reaction is considered stable. To determine state of this equilibrium, the reaction quotient should remain constant. With this tool, you can calculate the value of an equilibrium constant for a reaction while learning how to calculate the equilibrium constant with ease!

Below you can find the reversible reaction and equilibrium constant equations:

a[A] + b[B] ⇌ c[C] + d[D]

K = ([C]c * [D]d)/([B]b * [A]a)

where [A] and [B] are the molar concentrations of the reactants, and [C] and [D] are the molar concentrations of the products.

Calculating the value of the equilibrium constant for a reaction is helpful when determining the amount of each substance formed at equilibrium as a ratio of each other. The constant doesn't depend on initial concentrations of the reactants and products, as the same ratio will always be reached after a certain period of time. However the constant may be influenced by:

  • temperature
  • solvent
  • ionic strength

When is equilibrium constant used?

Equilibrium constants are useful if you want to understand biochemical processes such as oxygen transport by hemoglobin or acid-base homeostasis in humans. The changes in acid-base homeostasis are mainly reflected in changes in the arterial and venous blood pH. Doctors will also check the equilibrium constant of transferrin in the blood, as transferrin saturation is a symptom of iron deficiency anemia.

This equation helps explain what will be favored by the equilibrium - the reactants or the products. This can give important information about the nature of the reaction and its mechanism. You will find more on this topic below.

Equilibrium constant equation

The equilibrium constant of a reaction relates to all of the species present in the reaction. However, in this calculator, we assume that there is a maximum of two main reactants and two main products. For the hypothetical reaction:

a[A] + b[B] ⇌ c[C] + d[D]

the equilibrium constant equation has the following formula:

K = ([C]c * [D]d)/([B]b * [A]a)

The constant K reflect two measurements of quantity:

  • Kc - represents concentration, molarity, expressed as moles per liter (M=mol/L)
  • Kp - a function of both reactants and products partial pressure, usually in atmospheres, useful for calculations in the gas phase

If K>1 - equilibrium favours the products

If K<1 - equilibrium favours the reactants

If K=1 - the mixture contains similar amounts of both products and reactants at equilibrium

If you're not sure how to switch from moles to other units and the other way around, take a look at our mole calculator.

Let's calculate the value of the equilibrium constant for a reaction

To give you more insight about how this equation works in practice, we created this example.
You have a mixture of gaseous sulphur dioxide and oxygen, from which you can react to form sulphur trioxide. This is one of the steps in synthesising sulphuric acid:

2 SO₂ + O₂ ⇌ 2 SO₃

Therefore the equilibrium constant equation for this reaction is:

K = [SO₃]²/([SO₂]² * [O₂])

The reaction mixture is left for a while until an equilibrium is established. The reactants and products have the following concentrations:

  • SO₂: 0.03 mol/L
  • O₂: 0.035 mol/L
  • SO₃: 0.5 mol/L

When you put these numbers into the equation, K is found to be:

K = 0.05²/(0.03² * 0.035)

K = 7.937 * 10³

As K > 1 the equilibrium favours the products.

In our example, the concentrations of reactants and products at equilibrium were provided. We then used this information to calculate the equilibrium constant. But what if you knew the equilibrium constant and the unknown was the initial concentration or coefficient of a component? Well, don't worry! Our calculator works in reverse - so it solves both kinds of problem. Just input all of the data you have and the results will be compute for you in an instance.

How to calculate the equilibrium constant?

This paragraph mainly focuses on how the equilibrium constant is determined analytically. To calculate the value of the equilibrium constant for a reaction, you need to measure the concentrations of the reactants and/or products. There are both experimental and computational methods for constant evaluation. Among experimental methods, you can find:

  • potentiometry
  • spectrophotometry
  • NMR chemical shift
  • calorimetry

Within computational methods, there are:

  • chemical model
  • speciation calculations
  • refinement
  • model selection.

Although you already know how to calculate equilibrium constant, save yourself some time and make use of our calculator!

Dominika Śmiałek, MD, PhD candidate
a[A] + b[B] ⇌ c[C] + d[D]
Concentration: [A]
Coefficient: a
Concentration: [B]
Coefficient: b
Concentration: [C]
Coefficient: c
Concentration: [D]
Coefficient: d
Equilibrium constant
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