pKa & pKb Converter
Convert Ka to pKa or pKa back to Ka, convert Kb to pKb or pKb back to Kb, and find the full conjugate acid-base pair — Ka, pKa, Kb and pKb together — from just one known value, using pKa + pKb = 14 at 25°C, with acid/base strength classification and full step-by-step working.
Pick a conversion, then enter the known value.
pKa
Formula used: pKa = -log10(Ka)
pKa / pKb Strength Scale
Lower pKa or pKb means a stronger acid or base. See where your value sits from 0 to 14.
Step-by-Step pKa / pKb Conversion
Here's exactly how this answer was calculated, one step at a time.
Given: Ka = 1.8e-5
Step 1: Start from the definition of pKa
pKa is simply the negative base-10 logarithm of the acid dissociation constant, Ka.
pKa = -log10(Ka)Step 2: Substitute the given Ka
pKa = -log10(1.800 x 10⁻⁵) = 4.7447
Calculated pKa:
4.7447
pKa & pKb Converter: Convert Between Ka, pKa, Kb and pKb Instantly
This free pKa and pKb converter is built for chemistry students, lab technicians, and anyone who needs to move quickly between the different ways an acid or base's strength gets reported. It covers three real situations: converting a Ka value to pKa or a pKa value back to Ka, converting a Kb value to pKb or a pKb value back to Kb, and finding the complete conjugate acid-base pair — Ka, pKa, Kb, and pKb all at once — from just one known value.
Every result includes a plain-language strength classification (very strong, moderately strong, weak, very weak, or negligible), a visual strength scale, and a complete written solution showing exactly how the answer was reached. The conjugate pair mode is especially useful because textbooks and databases don't always list both the acid and base forms — this calculator fills in the missing half automatically using the relationship between Ka, Kb, and the water autoionization constant, Kw.
What Are Ka, Kb, pKa and pKb?
Ka, the acid dissociation constant, measures how far a weak acid's reaction HA <-> H+ + A- proceeds toward products at equilibrium. Kb, the base dissociation constant, measures the same thing for a weak base's reaction B + H2O <-> BH+ + OH-. Both are equilibrium constants, so both are usually small numbers written in scientific notation, like 1.8 x 10^-5 for acetic acid's Ka.
pKa and pKb are simply the negative base-10 logarithms of Ka and Kb. Taking a log converts an awkward, tiny scientific-notation number into a small, easy-to-compare positive number — typically somewhere between 0 and 14 for common weak acids and bases. This is exactly the same idea behind pH and pOH, which are the negative logs of [H+] and [OH-].
The Simple Math: How pKa and pKb Are Calculated
The formulas are short: pKa = -log10(Ka) and pKb = -log10(Kb). Reversing either equation to go back the other way gives Ka = 10^(-pKa) and Kb = 10^(-pKb). There's no equilibrium quadratic to solve here — unlike a weak acid or weak base pH calculation, converting between K and pK is a single logarithm, not a system that depends on concentration.
That's an important distinction to keep in mind: pKa and pKb are fixed properties of a particular acid or base at a given temperature, and they don't change with concentration the way pH does. A 0.1 M and a 0.001 M solution of acetic acid have very different pH values, but they share the exact same Ka and pKa.
Why Chemists Use pKa and pKb Instead of Ka and Kb
Comparing raw Ka or Kb values means squinting at scientific notation and mentally tracking negative exponents — is 4.3 x 10^-10 bigger or smaller than 6.2 x 10^-9? Converting to pKa turns this into a simple number comparison: 9.37 versus 8.21, where the lower number is instantly recognizable as the stronger acid.
This is why pKa and pKb tables are the standard reference format across organic chemistry, biochemistry, and pharmacology. A chemist scanning a table of pKa values for a series of related compounds can immediately rank their relative acidity at a glance, something that's much harder to do by comparing Ka values directly.
The Conjugate Acid-Base Relationship: pKa + pKb = 14
Every weak acid has a conjugate base, and every weak base has a conjugate acid. These pairs are linked by a clean relationship: Ka x Kb = Kw, where Kw is the water autoionization constant, equal to 1.0 x 10^-14 at 25°C. Taking the negative log of both sides turns this into an equally clean addition rule: pKa + pKb = 14.
This means if you know the Ka (or pKa) of an acid, you can instantly find the Kb (or pKb) of its conjugate base without any lab work — and the same works in reverse, from a base's Kb to its conjugate acid's Ka. This calculator's conjugate pair mode automates exactly this calculation, showing all four values side by side along with a check that pKa + pKb really does equal 14.
Reading a pKa Table: What Different Values Mean
A pKa below about 2 signals a very strong weak acid, dissociating heavily and bordering on strong-acid behavior. Common carboxylic acids like acetic acid and benzoic acid sit in the moderately strong range, roughly pKa 3 to 6, which is why vinegar and similar solutions are noticeably but not dangerously acidic. Values from about 7 to 12 describe weak acids that barely dissociate at typical concentrations, and anything above 12 is essentially non-acidic in water.
The same bands apply to pKb for bases, just read in the same direction: lower pKb means a stronger base. Ammonia's pKb of about 4.75 places it in the moderately strong weak-base range, while aniline's pKb near 9.4 makes it a distinctly weaker base, barely raising the pH of a solution it's dissolved in.
Common Mistakes When Converting Ka, Kb, pKa and pKb
The most common slip is direction confusion — remembering that a stronger acid has a smaller pKa, not a larger one, since the log relationship inverts the usual 'bigger number means more' intuition. Another frequent error is mixing up Ka and Kb (or pKa and pKb) for the same species — always double check whether a table is reporting the acid's own dissociation constant or its conjugate base's.
It's also easy to apply the pKa + pKb = 14 shortcut at the wrong temperature. That relationship depends on Kw = 1.0 x 10^-14, which is only accurate at 25°C; at other temperatures, Kw shifts slightly and the conjugate pair sum moves away from exactly 14. For everyday classroom and lab work at room temperature, though, the 14 rule holds reliably.
pKa and pKb in Drug Design and Biology
Pharmaceutical chemists rely heavily on a drug molecule's pKa to predict how it behaves in the body. Whether a drug is ionized or neutral at a given pH — determined directly by comparing its pKa to the surrounding pH using the Henderson-Hasselbalch equation — controls how well it crosses cell membranes, how it's absorbed in the stomach versus the intestine, and how it's filtered by the kidneys.
Biochemists use the pKa values of amino acid side chains to predict a protein's behavior at physiological pH, since whether a side chain is charged or neutral shapes protein folding, enzyme activity, and binding interactions. In both fields, pKa is treated as one of the first and most important properties to establish about any new molecule.
Common Acids, Bases and Their pKa/pKb Values
Acetic acid has a pKa of about 4.74, hydrofluoric acid about 3.18, benzoic acid about 4.19, and hydrocyanic acid a much higher 9.21, reflecting how weakly it dissociates. On the base side, ammonia has a pKb of about 4.74 (matching acetic acid's pKa is coincidental, not related), methylamine about 3.36, and aniline a noticeably higher pKb of about 9.37.
These reference values are what this converter's built-in acid and base presets are drawn from, so you can quick-fill a common compound instead of looking up its Ka or Kb elsewhere before converting. For compounds not on the list, any measured or literature Ka, Kb, pKa, or pKb value can be entered directly.
pKa & pKb Converter: Quick Reference Summary
Convert Ka to pKa with pKa = -log10(Ka), and reverse it with Ka = 10^(-pKa). Convert Kb to pKb with pKb = -log10(Kb), and reverse it with Kb = 10^(-pKb). For a conjugate acid-base pair at 25°C, use Ka x Kb = Kw = 1.0 x 10^-14, or the equivalent shortcut pKa + pKb = 14, to find one member of the pair from the other.
This free pKa and pKb converter is intended to support learning, lab planning, and quick reference lookups. Always confirm which species (the acid or its conjugate base) a table value refers to before converting, and for safety-critical, regulated, clinical, or industrial work, always confirm results against validated reference sources and your organisation's approved procedures.
Frequently Asked Questions
How do you convert Ka to pKa?
Use pKa = -log10(Ka). For example, acetic acid's Ka of 1.8 x 10^-5 gives pKa = -log10(1.8 x 10^-5) ≈ 4.74.
How do you convert pKa back to Ka?
Reverse the logarithm: Ka = 10^(-pKa). A pKa of 4.74 gives Ka = 10^(-4.74) ≈ 1.8 x 10^-5.
What is the relationship between pKa and pKb?
For a conjugate acid-base pair at 25°C, pKa + pKb = 14. This comes from Ka x Kb = Kw = 1.0 x 10^-14, the water autoionization constant.
Does a lower pKa mean a stronger or weaker acid?
A lower pKa means a stronger acid — it dissociates more completely into H+ and its conjugate base. The same logic applies to pKb and base strength.
How do I find the Kb of a conjugate base from an acid's Ka?
Use Kb = Kw / Ka, where Kw = 1.0 x 10^-14 at 25°C. This calculator's conjugate pair mode does this automatically and also reports pKb.
Is pKa the same as pH?
No. pKa is a fixed property of a particular acid, independent of concentration. pH depends on both the acid's Ka and its concentration in a specific solution. They're connected through the Henderson-Hasselbalch equation, pH = pKa + log10([A-]/[HA]).
Why do some pKa tables list negative values?
A negative pKa corresponds to a Ka greater than 1, meaning the acid dissociates almost completely — this is typical of strong acids, which are usually treated separately from the weak-acid Ka/pKa framework used here.
Does pKa + pKb = 14 always hold exactly?
It holds at 25°C, where Kw = 1.0 x 10^-14. At other temperatures, Kw changes slightly, so the exact sum shifts away from 14, though 14 remains the standard reference value used in most classroom and lab work.