Activity Coefficient Calculator

Created by Komal Rafay
Reviewed by Purnima Singh, PhD and Adena Benn
Last updated: Mar 10, 2023

For so many of us, chemistry and its related formulas are already too complicated, so why not make tools that make those complications disappear; that's how confident we are about our activity coefficient calculator.

We don't just have a tool but an informative article to go along with it. We discuss the activity coefficient of an ion in a dilute solution, the activity coefficient equation, and the significance of ionic strength and charge number of ions in determining the value for activity coefficient.

What is the activity coefficient?

In solutions, the activity coefficient measures the deviations of a solution from an ideal solution. And we can use the value of the activity coefficient to determine if the solution is ideal. There can be three possibilities:

  1. If the value of the activity coefficient of a mixture is equal to 1, it means it is an ideal solution.

  2. If the value is less than 1, it means the molecules of the mixture/solution are strongly bonded and would need more energy to dissociate them. This also indicates deviation from Raoult's law.

  3. If the value is greater than 1, it indicates the presence of dissolved gases in the solution.

At low-concentration solutions, you can also determine the activity coefficient using the Debye Huckel limiting law, which is why it is known by the name of "The Debye Huckel activity coefficient".

💡 Might we suggest trying out our Raoult's law calculator to understand its applications?

Activity coefficient calculator

Using the activity coefficient calculator is not difficult. As long as you input the correct values, you will get the perfect result.

You may ask, what are the things you need to input to get the activity coefficient value?

  • Input the ionic strength of the substance;
  • Next, input the charge number; and
  • As a result, you get the activity coefficient value.

Never did you ever think it would be this easy, right?

The most important thing to remember here is that an activity constant (A) determines the value of the activity coefficient, and this constant depends on the temperature of water used to make the solution. The value of the constant is 0.509 mol1/2kg1/20.509 \text { mol}^{-1/2}\text {kg}^{-1/2} with water at 25 °C25\ \degree \text C.

An advanced mode button in the calculator allows you to change the value of the constant. You may use it if you need the coefficient value at a different temperature.

Let's consider an example. For instance, if you enter the charge number as 33 and the ionic strength as 0.09 M0.09~ \text M, the result will be the activity coefficient value at 0.0420.042.

Activity coefficient formula

The ionic strength and charge number of ions are the two significant factors to help determine the activity coefficient of an ion in a dilute solution.

The formula on which the activity coefficient calculator is based is:

logf=Az2I,\log \text f = -\text A \text z^2 \sqrt \text I,


  • f\text f – Activity coefficient;
  • A\text A – Constant;
  • z\text z – Charge number; and
  • I\text I – Ionic strength.

The ionic strength represents the strength of the ions in the solution. We have an ionic strength calculator to help you know more about it.

A noteworthy thing to remember is that the activity coefficient formula is based on the Debye-Huckel limiting law, which assumes that the solutions used are very dilute (~0.01 M).

How to calculate the activity coefficient

No matter how many tools or software we have available at our disposal, it's always a good idea to be able to do the math and calculations yourself.

Now that we have the formula to calculate the Debye Huckel activity coefficient from the last section let's look at the steps:

  1. Determine the square root of the ionic strength (I).
  2. Calculate the square of the charge number (z).
  3. Multiply -1 by 0.509, the value of constant A.
  4. Determine the product of results from steps 1, 2, and 3.
  5. Estimate the anti-logarithm of the result of step 4.
  6. You have your activity coefficient value.

Significance of activity coefficient

The activity coefficient holds significance in the field of thermodynamics. It is a characteristic of a substance that helps determine the divergence of a solution from ideal behavior.

It helps measure the activity of a component of a mixture. It also signifies the chemical activity in a solution.

We have a solution dilution calculator if you are interested in knowing how to dilute a stock solution of known concentration to obtain an arbitrary volume of a diluted solution.


How can I calculate the Debye Huckel activity coefficient?

The formula for activity coefficient is the efficient way to determine the value for the activity coefficient.

log f = -Az²√I

So, let's follow the steps:

  1. Calculate the product of negative constant (-A), square of charge number (z). and square root of ionic strength (I).
  2. Take the log of results of step 1.
  3. You have the Debye Huckel activity coefficient.

Is ionic strength important for the activity coefficient?

Ionic strength is significant in determining the activity coefficient of an ion, as it is part of the formula.

log f = -Az²√I


  • f – Activity coefficient;
  • A – Constant;
  • z – Charge number; and
  • I – Ionic strength.

The activity coefficient equation also tells us that the ionic strength and activity coefficient are inversely proportional to each other.

What is the significance of activity coefficient?

The activity coefficient lets you evaluate the solutes' effects on the properties of the solution.

Since the activity coefficient is a function of concentration, Debye and Huckel used it to test their theory of calculating single-ion activity coefficients.

What does activity coefficient depend on?

In terms of calculating its value, the activity coefficient depends on ionic strength and the charge number of ions.

It also depends on the number of ions the solute dissociates in the solution when dealing with electrolytes.

What is the meaning of activity coefficient greater than 1?

Generally, the value for the activity coefficient is one or less than one for ions in an aqueous solution.

The instances where its value is greater than one implies the solution has dissolved gases.

What is the activity coefficient if ionic strength is 0.1?

The activity coefficient value is 0.227 when the ionic strength is equal to 0.1 and an arbitrary value of charge number is equal to 2.

Komal Rafay
Charge number of ion (z)
Ionic strength (I)
Activity coefficient (f)
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