pKa Calculator

mol/L

Calculation Results

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Common Acids Reference

Acid Formula Ka Value pKa
Acetic Acid CH₃COOH 1.8 × 10⁻⁵ 4.74
Formic Acid HCOOH 1.8 × 10⁻⁴ 3.74
Hydrochloric Acid HCl ~10⁷ -7
Carbonic Acid H₂CO₃ 4.3 × 10⁻⁷ 6.37
Phosphoric Acid H₃PO₄ 7.5 × 10⁻³ 2.12

The pKa value is calculated from the acid dissociation constant (Ka) using the formula:

pKa = -log₁₀(Ka)

Where:

  • pKa is the negative logarithm of the acid dissociation constant
  • Ka is the acid dissociation constant, representing the equilibrium between the acid and its conjugate base

A lower pKa value indicates a stronger acid, while a higher pKa value indicates a weaker acid. The pKa scale typically ranges from -10 for very strong acids to 50 for very weak acids.

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What is a pKa Calculator?

The pKa calculator helps you determine the acid dissociation constant (Ka) and pKa of a substance based on your known parameters — pH, Ka, or molar concentrations of acid and base.
This tool simplifies the often confusing chemical equilibrium equations into quick, precise results for students, chemists, and researchers.

Using this pKa calculator online, you can:

  • Calculate pKa from Ka
  • Find pKa from pH using the Henderson-Hasselbalch equation
  • Convert pKa to Ka
  • Compute pH from pKa
  • Explore pKa from structure or SMILES (for advanced chemical modeling)

Understanding pKa is essential for predicting acid-base reactions, buffer capacities, and molecular stability in organic and inorganic chemistry.

Understanding pKa — The Key to Acid Strength

In chemistry, pKa measures how strongly an acid holds onto its proton (H⁺). It is defined as:

pKa = -log10(Ka)

A lower pKa indicates a stronger acid because it donates protons more easily. Conversely, a higher pKa represents a weaker acid.

To visualize this:

  • Strong acids → pKa < 1
  • Moderate acids → pKa ≈ 3–6
  • Weak acids → pKa > 10

This relationship between pKa and Ka helps chemists predict reaction directions and buffer capacities in any solution.

If you’re working with atomic-level data, our Atom Calculator can help you determine the structure and atomic count in molecules before running pKa calculations.

pKa Table – Common Acids and Their pKa Values

Functional GroupFormulapKa Value
Hydroiodic acidHI-10
Hydrobromic acidHBr-9
Hydrochloric acidHCl-6
Sulfuric acidH₂SO₄-3
Hydrofluoric acidHF3.2
Carboxylic acidR–COOH4
Protonated aminesR–NH₃⁺9–11
WaterH₂O14
AlcoholCH₃CH₂OH17
Ketone / AldehydeR–CO–R’20–24
AlkaneCnH₂n₊₂~50

These values are vital when designing buffer systems or predicting reaction outcomes. For example, carboxylic acids (pKa ≈ 4) are much stronger acids than alcohols (pKa ≈ 17).

Relationship Between pKa and pH

The pKa calculator with steps uses the Henderson-Hasselbalch equation to connect pH and pKa values:

pH = pKa + log10([A⁻]/[HA])

Where:

  • [A⁻] = molar concentration of the conjugate base
  • [HA] = molar concentration of the weak acid

This equation helps determine the pH of a buffer or the relative strength of an acid/base pair.

Interpretation

  • When [A⁻] = [HA], then pH = pKa
  • When [HA] > [A⁻], then pH < pKa (acidic condition)
  • When [A⁻] > [HA], then pH > pKa (basic condition)

The closer the pH is to the pKa, the stronger the buffering capacity of a solution.
If you’re measuring pH directly, try our pH Calculator to quickly evaluate solution acidity before using it in the pKa formula.

Relationship Between pKa and Ka

The acid dissociation constant (Ka) measures the degree of acid ionization in a solution:

Ka = [H⁺][A⁻] / [HA]

Since Ka values vary widely, scientists use pKa for convenience by taking the negative logarithm of Ka:

pKa = -log10(Ka)

The relationship between the two is inverse — as Ka increases, pKa decreases. This makes strong acids have large Ka but small pKa values.

To convert between them, our Ka from pKa calculator uses the formula:

Ka = 10^(-pKa)

This is particularly useful when performing equilibrium calculations or determining the strength of weak acids.

How to Calculate pKa from pH – Step-by-Step

Let’s go through how you can calculate pKa from pH using the Henderson-Hasselbalch equation.

Formula:
pKa = pH − log10([A⁻]/[HA])

Example:
Find the pKa of a solution with:

  • pH = 4.8
  • [CH₃COOH] = 0.1 M
  • [CH₃COONa] = 0.01 M

Solution:
pKa = 4.8 − log10(0.01 / 0.1)
pKa = 4.8 − (−1)
pKa = 5.8

Therefore, the pKa = 5.8.

This process shows the logic behind our pKa calculator with steps, which instantly performs this calculation when you input the given concentrations.

To understand molecular structure contributions in these reactions, you can use the Bond Order Calculator to analyze electron pair bonding strength before assessing acid dissociation.

How to Calculate pKa from Ka

You can also determine pKa directly from the acid dissociation constant (Ka) using:

pKa = -log10(Ka)

Example:
If Ka = 1.5 × 10⁻⁵
Then, pKa = -log10(1.5 × 10⁻⁵) = 4.82

Alternatively, to find Ka from pKa, use:

Ka = 10^(-pKa)

If pKa = 9.16
Then, Ka = 10⁻⁹.¹⁶ = 6.9 × 10⁻¹⁰

This relationship is automatically computed in the pKa calculator online, so you can skip the manual logarithmic math.

How to Use the pKa Calculator

Our pKa calculator has two primary functions:

  1. pKa from pH
    Enter:
    • pH value
    • [HA] (weak acid concentration)
    • [A⁻] (conjugate base concentration)
      Then the calculator applies the Henderson-Hasselbalch formula.
  2. pKa from Ka
    Enter the known Ka, and the tool will compute pKa instantly using the -log10 relationship.

You can also explore pKa calculator from structure or pKa calculator from SMILES for complex organic compounds — especially if you’re modeling molecules computationally or comparing functional group strengths.

If you’re studying atomic or molecular properties related to acid-base balance, try our Electron Configuration Calculator to visualize how electrons contribute to reactivity.

Practical Applications of pKa

Understanding and calculating pKa is essential across various chemical disciplines:

  • Organic Chemistry: To predict nucleophilicity, electrophilicity, and reaction outcomes.
  • Biochemistry: For enzyme catalysis and protein folding based on amino acid side-chain pKa values.
  • Pharmaceutical Chemistry: To determine drug solubility and absorption (pKa influences bioavailability).
  • Environmental Chemistry: To estimate acid rain impact and buffer capacity of natural waters.

For deeper exploration into atomic mass relationships, check our Atomic Mass Calculator and Average Atomic Mass Calculator, which complement pKa calculations in stoichiometric experiments.

Examples of pKa Calculation

Example 1 – From pH

Given:

  • pH = 3.38
  • [HC₃H₅O₃] = 0.75 M (lactic acid)
  • [C₃H₅O₃⁻] = 0.25 M (sodium lactate)

pKa = 3.38 − log10(0.25 / 0.75)
pKa = 3.38 − (−0.477)
pKa = 3.85

Example 2 – From Ka

Given:

  • Ka = 6.8 × 10⁻¹⁰

pKa = -log10(6.8 × 10⁻¹⁰)
pKa = 9.162

These examples demonstrate how the Ka from pKa calculator and pH from pKa calculator operate in practice — fast, accurate, and suitable for both academic and lab use.

FAQs about pKa Calculations

1. What is the difference between pKa and Ka?
Ka measures how easily an acid dissociates in water. pKa is simply the negative logarithm of Ka. Both express acid strength, but pKa is easier to interpret because it uses manageable numbers.

2. Are pH and pKa the same?
No. pH measures the actual hydrogen ion concentration in a solution, while pKa indicates how readily an acid donates its hydrogen.

3. How do I calculate pKa from Ka?
Use pKa = -log10(Ka). For example, Ka = 1.8 × 10⁻⁵ → pKa = 4.74.

4. How do I calculate pKa from pH?
Use pKa = pH − log10([A⁻]/[HA]) and input the concentrations.

5. What does pKa tell you?
It indicates how strong or weak an acid is. Lower pKa = stronger acid; higher pKa = weaker acid.

6. What is a good buffer range?
A buffer is most effective when pH ≈ pKa ± 1.

The pKa calculator is an essential chemistry tool that bridges the relationship between pH, Ka, and acid strength. Whether you’re studying equilibrium reactions, designing a buffer, or exploring acid-base chemistry, this calculator simplifies the process.

With built-in options to find Ka from pKa, pH from pKa, or even pKa from SMILES, it supports both academic learning and professional research.

Pair it with related tools like the Atom Calculator, Bond Order Calculator, and Electron Configuration Calculator to deepen your understanding of chemical reactions from atomic to molecular scale.