This pK_a calculator will help you determine the pK_a value in two ways: from a specific pH with the Henderson-Hasselbalch equation or from the acid dissociation constant (K_a).

In this article, you will find information on:

• The definition of pK_a;
• How to use the pK_a table;
• Relationship between pK_a and pH;
• Relationship between pK_a and K_a; and
• Useful examples are also provided to help you calculate pK_a values like a pro!

You have probably seen the term pKa before from your chemistry class in high school🧑‍🔬​. What is it exactly? Do you recall?

It’s very simple! The pK_a determines how weak or strong an acid is. To be more precise, pK_a tells you how strongly the Brønsted acid holds on a given proton (H⁺). It allows you to predict how each acid and base solution will react in a specific experimental setting.

Interpretation of pK_a

The lower the pK_a, the stronger the acid. This means:

• The H⁺ is held more loosely by the acid; and
• The acid can give up on H⁺ more easily.

The higher the pK_a, the weaker the acid. This means:

• The H⁺ is held more tightly by the acid; and
• The acid does not easily donate a H⁺.

Before going through equations and calculations of pK_a, here is the easiest way to find the pK_a of a compound — by using the pK_a table.

This table can be used when you are trying to make a buffer or carry out a reaction from scratch. It guides the selection of acids and bases in a reaction, since knowing the pK_a values of each compound will predict their reactive behavior 🧪​. So, if you don’t know the pH or the K_a of your compounds, check it out!

You can find a shortened version of the pK_a table with the most common functional groups used in basic chemistry below:

For more visualization, check out Master Organic Chemistry website, which provides a clear structural illustration of each functional group and the conjugate base.

Henderson-Hasselbalch equation

The pK_a calculator is based on the well-known Henderson-Hasselbalch equation, providing the relationship between pH and pK_a.

where:

• 
  ${\mathrm{A}}^{-}$

   

   — Molar concentration of the conjugate base; and
• 
  $\mathrm{H}\mathrm{A}$

   

   — Molar concentration of the weak acid.

Relationship of pK_a and pH

• If 

  $[\mathrm{H}\mathrm{A}]=[{\mathrm{A}}^{-}]$

  , then 

  $\mathrm{l}\mathrm{o}\mathrm{g}\frac{[{\mathrm{A}}^{-}]}{[\mathrm{H}\mathrm{A}]}=0$

  :

  When molar concentrations of weak acid and conjugate base are the same, the logarithm is exactly 0. This means that 

  $\mathrm{p}\mathrm{H}=\mathrm{p}{\mathrm{K}}_{\mathrm{a}}$

  ​.

• If 

  $[\mathrm{H}\mathrm{A}]>[{\mathrm{A}}^{-}]$

  , then 

  $\mathrm{l}\mathrm{o}\mathrm{g}\frac{[{\mathrm{A}}^{-}]}{[\mathrm{H}\mathrm{A}]}<0$

  :

  When the molar concentration of the weak acid is higher than that of the conjugate base, the logarithm is negative. Thus, 

  $\mathrm{p}\mathrm{H}<\mathrm{p}{\mathrm{K}}_{\mathrm{a}}$

  ​.

• If 

  $[\mathrm{H}\mathrm{A}]<[{\mathrm{A}}^{-}]$

  , then 

  $\mathrm{l}\mathrm{o}\mathrm{g}\frac{[{\mathrm{A}}^{-}]}{[\mathrm{H}\mathrm{A}]}>0$

  :

  When the molar concentration of the conjugate base is higher than that of the weak acid, the logarithm is positive. Thus, 

  $\mathrm{p}\mathrm{H}>\mathrm{p}{\mathrm{K}}_{\mathrm{a}}$

  ​.

Here is another terminology to recall from your chemistry lecture 🤓​ — the acid dissociation constant (K_a), also known as acid ionization constant.

K_a is a constant value measured at equilibrium, indicating how acids dissociate in a solution. The higher K_a values, the stronger the acid and the easier the dissociation (H⁺ donating) from the other components.

Relationship of pK_a and K_a

pK_a is negatively correlated to K_a, meaning that if one value increases ⬆️, the other value decreases​ ⬇️. ​ Basically, K_a is simply the logarithm of pK_a:

You can also write the equation as follows in case you want to calculate an unknown K_a from pK_a:

If you are interested in knowing more about what is a logarithm and how to solve the log of a value, the log calculator will be very helpful!

Acid dissociation equation

This equation explains the dissociation of an acid at equilibrium, giving hydronium ions (H⁺) and conjugate base (A^–) as products. Knowing how to write the reaction in this form is very important to make sure you know which compound is the nominator or denominator when using the Henderson-Hasselbalch equation to calculate pK_a or pH.

Hence, K_a can be represented by the concentration of products to the concentration of the reactant:

✏️​Practice makes progress! Explore the provided examples below to improve your knowledge of pK_a even more.

Calculating pK_a from pH

Example 1 — Calculate the pK_a of a solution containing 

 of acetic acid and 

 of acetate ion. Note that 

.

1. Write the acid dissociation equation of this reaction to identify the weak acid and conjugate base:

2. Calculate the pK_a using the Henderson-Hasselbalch equation:

Example 2 — Calculate the pK_a of a solution containing 

 of lactic acid and 

 of sodium lactate. Note that 

.

1. Write the acid dissociation equation of this reaction to identify the weak acid and conjugate base:

2. Calculate the pK_a using the Henderson-Hasselbalch equation:

Phew! 😮‍💨​​ That was a bit tiring, wasn’t it? Let RevisionTown Calculator’s pK_a calculator make things a lot easier for you — easily fill in your given pH, conjugate base concentration and weak acid concentration in the section pK_a from pH.

Calculating pK_a from K_a

Example 3 — If 

, how much is pK_a?

Example 4 — If 

, how much is pK_a?

Otherwise, feel free to input the K_a value in the section pK_a from K_a in our pK_a calculator and get your result in one second!