pKa to Ka Converter

The pKa to Ka Converter converts between the four constants that describe acid and base strength — Ka, pKa, Kb, and pKb. Enter any one of them and the tool returns the other three using two simple relationships: the definition pKa = −log₁₀(Ka) and the conjugate-pair rule pKa + pKb = pKw. It also places your acid on a colour-coded strength spectrum, tells you how strong its conjugate base is, and estimates the percent ionization in a 0.1 M solution.

How to Convert pKa to Ka

The acid dissociation constant Ka and its logarithmic form pKa are two ways of writing the same thing. pKa is just the negative base-10 logarithm of Ka, which compresses the huge range of Ka values (from about 10⁹ for strong acids to 10⁻⁵⁰ for very weak ones) onto a convenient scale that mostly runs from about −10 to 16.

For example, acetic acid has Ka = 1.74 × 10⁻⁵. Taking the negative log gives pKa = −log₁₀(1.74 × 10⁻⁵) = 4.76. Reversing it, Ka = 10⁻⁴·⁷⁶ = 1.74 × 10⁻⁵. The same pair of formulas converts Kb to pKb for bases.

The Conjugate-Pair Relationship: Ka, Kb, and Kw

Every acid has a conjugate base, and their strengths are linked through water. The product of the acid constant and the conjugate base constant always equals the ion-product of water:

This is why the converter can give you all four values from a single input: it finds pKa first, then derives pKb = pKw − pKa, and finally exponentiates back to Ka and Kb. It also explains the famous see-saw rule — the stronger an acid, the weaker its conjugate base, and vice versa.

Worked Example: Acetic Acid

What the Numbers Tell You About Acid Strength

Common Acids and Their pKa / Ka Values

Why pKw Is Adjustable

The value pKw = 14.00 only holds at 25 °C. Because the self-ionization of water absorbs heat, Kw rises as water warms and falls as it cools, so pKw drops to about 13.26 at 50 °C and rises to roughly 14.95 at 0 °C. If you are working at a non-standard temperature, open the advanced option and enter the matching pKw so the Kb and pKb results stay accurate.

Key Points to Remember

How to Use This Converter

1. Choose what you know: Select Ka, pKa, Kb, or pKb from the dropdown.
2. Enter the value: Type a plain number for pKa or pKb (e.g. 4.76), or scientific notation for Ka or Kb (e.g. 1.8e-5).
3. (Optional) Set pKw: Open the advanced option to change pKw if you are not at 25 °C.
4. Click Convert: Read off all four constants, the strength spectrum, the conjugate relationship, the percent ionization, and the full working.

Frequently Asked Questions

How do you convert pKa to Ka?

pKa is the negative base-10 logarithm of Ka, so Ka = 10⁻ᵖᴷᵃ. For example, a pKa of 4.76 gives Ka = 10⁻⁴·⁷⁶ = 1.74 × 10⁻⁵. To go the other way, pKa = −log₁₀(Ka).

What is the relationship between Ka and Kb?

For a conjugate acid–base pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C. In logarithmic form this is pKa + pKb = pKw = 14. So once you know one constant you can find all four.

What does a small Ka or large pKa mean?

A small Ka (large pKa) means the acid only ionizes slightly in water, so it is a weak acid. A large Ka (small or negative pKa) means the acid ionizes almost completely and is a strong acid. The weaker the acid, the stronger its conjugate base.

What is pKw and why is it 14?

pKw is the negative log of the water ion-product Kw. At 25 °C, Kw = 1.0 × 10⁻¹⁴, so pKw = 14.00. Because Kw rises with temperature, pKw falls slightly when water is warmer, which is why this converter lets you change it.

How do I find Kb from pKa?

First convert pKa to pKb using pKb = pKw − pKa (so 14 − pKa at 25 °C). Then Kb = 10⁻ᵖᴷᵇ. For acetic acid, pKa = 4.76 gives pKb = 9.24 and Kb = 5.75 × 10⁻¹⁰ for its conjugate base, acetate.

Is Ka the same as the acid strength?

Yes. Ka is the equilibrium constant for an acid donating a proton to water, so a larger Ka means the equilibrium lies further toward the ionized products and the acid is stronger. Because pKa = −log₁₀(Ka), a lower pKa also means a stronger acid.

Additional Resources

• Acid Dissociation Constant - Wikipedia
• Self-ionization of Water - Wikipedia