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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 calculationFAQs

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

In this article, you will find information on:

  • The definition of pKa;
  • How to use the pKa table;
  • Relationship between pKa and pH;
  • Relationship between pKa and Ka; and
  • Useful examples are also provided to help you calculate pKa 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 pKa determines how weak or strong an acid is. To be more precise, pKa 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 pKa

The lower the pKa, 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 pKa, 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 pKa, here is the easiest way to find the pKa of a compound — by using the pKa 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 pKa values of each compound will predict their reactive behavior 🧪​. So, if you don't know the pH or the Ka of your compounds, check it out!

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

Functional group



Hydroiodic acid



Hydrobromic acid



Hydrochloric acid



Sulfuric acid



Hydronium ion



Sulfonic acids



Hydrofluoric acid



Carboxylic acid



Protonated amines















Ketone / Aldehyde
























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

pKa and pH — How to calculate pKa from pH?

Henderson-Hasselbalch equation

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

pH=pKa+log10[A][HA]\text{pH} = \mathrm{pK_a} + \mathrm{log_{10}\frac{[A^-]}{[HA]}}


  • A\mathrm{A^-} — Molar concentration of the conjugate base; and
  • HA\mathrm{HA} — Molar concentration of the weak acid.

Relationship of pKa and pH

  • If [HA]=[A]\small\mathrm{[HA] = [A^-]}, then log[A][HA]=0\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 pH=pKa\small\mathrm{pH = pK_a}.

  • If [HA]>[A]\small\mathrm{[HA] > [A^-]}, then log[A][HA]<0\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, pH<pKa\small\mathrm{pH < pK_a}.

  • If [HA]<[A]\small\mathrm{[HA] < [A^-]}, then log[A][HA]>0\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, pH>pKa\small\mathrm{pH > pK_a}.

💡 Did you know that the buffer capacity increases as the value of pH and pKa 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 (Ka), also known as acid ionization constant.

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

Relationship of pKa and Ka

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

pKa=log10[Ka]\rm {pK_a} = {-log_{10}}[{K_a}]

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

Ka=10pKa\rm {K_a} = 10^{-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, Ka does not vary with the concentration, but it does vary with temperature changes. That means Ka 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 Henderson-Hasselbalch equation to calculate pKa or pH.

HAH++A\rm HA \rightleftharpoons H^{+} + A^{-}

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

Ka=[H+][A]HA\rm K_a = \small\frac{[H^+][A^-]}{HA}

Examples of pKa calculation

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

Calculating pKa from pH

Example 1 — Calculate the pKa of a solution containing 0.1 M\small0.1\ \mathrm{M} of acetic acid and 0.01 M\small0.01\ \mathrm{M} of acetate ion. Note that pH=4.8\small\mathrm{pH} = 4.8.

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

HA H++ACH3COOH H3++CH3COO\footnotesize \begin{align*} \rm HA\ &\rm\rightleftharpoons H^{+} + A^{-} \\ \rm CH_3COOH\ &\rm\rightleftharpoons H^{+}_3 + CH_3COO^{-} \end{align*}

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

pH =pKa+log10[CH3COO][CH3COOH]pKa =log10[CH3COO][CH3COOH]pHpKa =log10[0.01][0.1]4.8pKa =14.8pKa =5.8pKa =5.8\footnotesize \begin{align*} \rm \text{pH}\ &\rm= \mathrm{pK_a} + \mathrm{log_{10}\frac{[CH_3COO^-]}{[CH_3COOH]}} \\[1.5em] -\rm{pK_a}\ &\rm= \mathrm{log_{10}\frac{[CH_3COO^-]}{[CH_3COOH]}} - \mathrm{pH} \\[1.5em] -\rm{pK_a}\ &\rm= \mathrm{log_{10}\frac{[0.01]}{[0.1]}} - 4.8 \\[1.5em] -\rm{pK_a}\ &\rm= -1 - 4.8 \\[.7em] -\rm{pK_a}\ &\rm= -5.8 \\[.7em] \rm{pK_a}\ &\rm= 5.8 \end{align*}

Example 2 — Calculate the pKa of a solution containing 0.75 M\small0.75\ \mathrm{M} of lactic acid and 0.25 M\small0.25\ \mathrm{M} of sodium lactate. Note that pH=3.38\small\mathrm{pH} = 3.38.

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

HA H++AHC3H5O3 H++C3H5O3\footnotesize \begin{align*} \rm HA\ &\rm\rightleftharpoons H^{+} + A^{-} \\ \rm HC_3H_5O_3\ &\rm\rightleftharpoons H^{+} + C_3H_5O^{-}_3 \end{align*}

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

pH =pKa+log10[C3H5O3][HC3H5O3]pKa =log10[C3H5O3][HC3H5O3]pHpKa =log10[0.25][0.75]3.38pKa =0.0473.38pKa =3.85pKa =3.85\footnotesize \begin{align*} \text{pH}\ &\rm= \rm{pK_a} + \mathrm{log_{10}\frac{[C_3H_5O_3^-]}{[HC_3H_5O_3]}} \\[1.5em] -\rm{pK_a}\ &\rm= \rm{log_{10}\frac{[C_3H_5O_3^-]}{[HC_3H_5O_3]}} - \rm{pH} \\[1.5em] -\rm{pK_a}\ &\rm= \rm{log_{10}\frac{[0.25]}{[0.75]}} - 3.38 \\[1.5em] -\rm{pK_a}\ &\rm= -0.047 - 3.38 \\[.7em] -\rm{pK_a}\ &\rm= -3.85 \\[.7em] \rm{pK_a}\ &\rm= 3.85 \end{align*}

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

Calculating pKa from Ka

Example 3 — If Ka=1.5×105\small\mathrm{K_a} = 1.5 × 10^{-5}, how much is pKa?

pKa=log10[Ka]=log10[1.5×105]=3.824\footnotesize \begin{align*} \mathrm{pK_a} &= \mathrm{-log_{10}}[\mathrm{K_a}] \\ &= \mathrm{-log_{10}}[\mathrm1.5 × 10^{-5}] \\ &= 3.824 \end{align*}

Example 4 — If Ka=6.8×1010\small\mathrm{K_a} = 6.8 × 10^{-10}, how much is pKa?

pKa=log10[Ka]=log10[6.8×1010]=9.162\footnotesize \begin{align*} \mathrm{pK_a} &= \mathrm{-log_{10}}[\mathrm{K_a}] \\ &= \mathrm{-log_{10}}[\mathrm 6.8 × 10^{-10}] \\ &= 9.162 \end{align*}

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


What is the difference between pKa and Ka?

Ka is the acid dissociation constant, which determines how strong an acid is by its ability to dissociate in a solution. pKa, on the other hand, is basically the negative log of Ka. Both of these values can determine how strong or weak an acid is.

Are pH and pKa the same?

No, pH and pKa 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 pKa, it tells us how strong an acid is.

How do I calculate pKa from pH?

Use the Henderson-Hasselbalch equation to calculate pKa from pH:

pH = pKa + log10[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 pKa.

How do I calculate pKa from Ka?

Use the relationship between pKa and the acid dissociation constant (Ka): pKa = -log10[Ka]. The equation can also be reverted in case pKa is given to calculate Ka: Ka = 10-pKa.

How do I calculate pKa of 0.58 M sodium acetate and 1.0 M acetic acid?

To calculate pKa from pH:

  1. Apply the Henderson-Hasselbalch equation:

    pH = pKa + log10[A-]/[HA]

  2. The conjugate base, sodium acetate, is [A-]: [C2H3NaO2]

  3. The weak acid, acetic acid, is [HA]: [CH3COOH].

  4. Thus, considering pH = 4.5, we can calculate the pKa as follows:

    pH = pKa + log[C2H3NaO2] / [CH3COOH]

    4.5 = pKa + log[0.58]/[1.0]

    pKa = -0.236 − 4.5 = 4.737

  5. Congratulations! Now, you can check your answer using Omni Calculator's pKa calculator.

How much is pKa of acetic acid if the Ka is 1.8×10⁻⁵?

The pKa of acetic acid is 4.745. Given the Ka of 1.8 × 10-5, pKa can be calculated as follows:

pKa = -log10[Ka]
pKa = -(-4.745) = 4.745

pKa from pH

pKa from Ka

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