pKa Calculator
Table of contents
What is pKa?pKa tablepKa and pH — How to calculate pKa from pH?pKa and Ka — How to calculate pKa from Ka?Examples of pKa calculationFAQsThis pK_{a} calculator will help you determine the pK_{a} value in two ways: from a specific pH with the HendersonHasselbalch 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!
What is pKa?
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^{+}.
💡 If you are looking for a way to calculate the pH of a solution in your experiment ⚗️, knowing the concentration of the acid or base is very important. You might find Omni's pH calculator and concentration calculator helpful!
pKa table
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
.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:
Functional group  Formula  pK_{a} 

Hydroiodic acid  HI  10 
Hydrobromic acid  HBr  9 
Hydrochloric acid  HCl  6 
Sulfuric acid  H_{2}SO_{4}  3 
Hydronium ion  H_{3}O^{+}  1.7 
Sulfonic acids  R–SO_{3}H  1 
Hydrofluoric acid  HF  3.2 
Carboxylic acid  R–COOH  4 
Protonated amines  R–NH_{2}  911 
Thiols  R–SH  13 
Malonates  CH_{2}(COOH)_{2}  13 
Water  H_{2}O  14 
Alcohol  CH_{3}CH_{2}OH  17 
Ketone / Aldehyde  R–CH_{2}O / R–CHO  2024 
Nitrile  R–C≡N  25 
Ester  R–COO–R'  25 
Alkyne  R–C≡C–R'  25 
Amines  R–NH_{2}  ~35 
Hydrogen  H  36 
Alkene  R–C=C–R'  ~43 
Alkane  C_{n}H_{2n+2}  ~50 
For more visualization, check out
, which provides a clear structural illustration of each functional group and the conjugate base.pKa and pH — How to calculate pKa from pH?
HendersonHasselbalch equation
The pK_{a} calculator is based on the wellknown HendersonHasselbalch equation, providing the relationship between pH and pK_{a}.
where:
 $\mathrm{A^}$ — Molar concentration of the conjugate base; and
 $\mathrm{HA}$ — Molar concentration of the weak acid.
Relationship of pK_{a} and pH

If $\small\mathrm{[HA] = [A^]}$, then $\mathrm{log\large\frac{[A^]}{[HA]} \small = 0}$:
When molar concentrations of weak acid and conjugate base are the same, the logarithm is exactly 0. This means that $\small\mathrm{pH = pK_a}$.

If $\small\mathrm{[HA] > [A^]}$, then $\mathrm{log\large\frac{[A^]}{[HA]}\small < 0}$:
When the molar concentration of the weak acid is higher than that of the conjugate base, the logarithm is negative. Thus, $\small\mathrm{pH < pK_a}$.

If $\small\mathrm{[HA] < [A^]}$, then $\mathrm{log\large\frac{[A^]}{[HA]}\small > 0}$:
When the molar concentration of the conjugate base is higher than that of the weak acid, the logarithm is positive. Thus, $\small\mathrm{pH > pK_a}$.
💡 Did you know that the buffer capacity increases as the value of pH and pK_{a} are closer together? This allows a buffer to maintain its pH range despite the addition of a stronger acid or base. Check our buffer capacity calculator for more information! 💬
pKa and Ka — How to calculate pKa from Ka?
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!
💡 Unlike pH, K_{a} does not vary with the concentration, but it does vary with temperature changes. That means K_{a} values of acids are usually fixed.
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 HendersonHasselbalch equation to calculate pK_{a} or pH.
Hence, K_{a} can be represented by the concentration of products to the concentration of the reactant:
Examples of pKa calculation
✏️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 $\small0.1\ \mathrm{M}$ of acetic acid and $\small0.01\ \mathrm{M}$ of acetate ion. Note that $\small\mathrm{pH} = 4.8$.
1. Write the acid dissociation equation of this reaction to identify the weak acid and conjugate base:
2. Calculate the pK_{a} using the HendersonHasselbalch equation:
Example 2 — Calculate the pK_{a} of a solution containing $\small0.75\ \mathrm{M}$ of lactic acid and $\small0.25\ \mathrm{M}$ of sodium lactate. Note that $\small\mathrm{pH} = 3.38$.
1. Write the acid dissociation equation of this reaction to identify the weak acid and conjugate base:
2. Calculate the pK_{a} using the HendersonHasselbalch equation:
Phew! 😮💨 That was a bit tiring, wasn't it? Let Omni 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 $\small\mathrm{K_a} = 1.5 × 10^{5}$, how much is pK_{a}?
Example 4 — If $\small\mathrm{K_a} = 6.8 × 10^{10}$, 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!
What is the difference between pKa and Ka?
K_{a} is the acid dissociation constant, which determines how strong an acid is by its ability to dissociate in a solution. pK_{a}, on the other hand, is basically the negative log of K_{a}. Both of these values can determine how strong or weak an acid is.
Are pH and pKa the same?
No, pH and pK_{a} are two different things. pH is a scale that measures the presence of H^{+} ions in a solution, making it acidic, neutral, or basic. As for pK_{a}, it tells us how strong an acid is.
How do I calculate pKa from pH?
Use the HendersonHasselbalch equation to calculate pK_{a} from pH:
pH = pK_{a} + log_{10}[A^{}]/[HA]
where [A^{}] is the conjugate base and [HA] is the weak acid. The pH and molar concentrations of the acid and base must be known to calculate pK_{a}.
How do I calculate pKa from Ka?
Use the relationship between pK_{a} and the acid dissociation constant (K_{a}): pK_{a} = log_{10}[K_{a}]. The equation can also be reverted in case pK_{a} is given to calculate K_{a}: K_{a} = 10^{pKa}.
How do I calculate pKa of 0.58 M sodium acetate and 1.0 M acetic acid?
To calculate pK_{a} from pH:

Apply the HendersonHasselbalch equation:
pH = pK_{a} + log_{10}[A^{}]/[HA]

The conjugate base, sodium acetate, is [A^{}]: [C_{2}H_{3}NaO_{2}]

The weak acid, acetic acid, is [HA]: [CH_{3}COOH].

Thus, considering pH = 4.5, we can calculate the pK_{a} as follows:
pH = pK_{a} + log[C_{2}H_{3}NaO_{2}] / [CH_{3}COOH]
4.5 = pK_{a} + log[0.58]/[1.0]
pK_{a} = 0.236 − 4.5 = 4.737

Congratulations! Now, you can check your answer using Omni Calculator's pK_{a} calculator.
How much is pKa of acetic acid if the Ka is 1.8×10⁻⁵?
The pK_{a} of acetic acid is 4.745. Given the K_{a} of 1.8 × 10^{5}, pK_{a} can be calculated as follows:
pK_{a} = log_{10}[K_{a}]
pK_{a} = (4.745) = 4.745