pKa ⇄ Ka Converter
Convert between an acid dissociation constant Ka and its pKa in either direction — the logarithmic relationship at the heart of acid strength.
How it works
pKa = −log₁₀(Ka) · Ka = 10^(−pKa)
The acid dissociation constant Ka measures how completely an acid ionizes in water — larger Ka means a stronger acid. Because Ka spans many orders of magnitude, chemists usually work with its negative base-10 logarithm, pKa, which puts most acids on a convenient scale of about −2 to 14. A lower pKa means a stronger acid, and each unit of pKa is a tenfold change in Ka.
Worked example
Acetic acid has Ka = 1.8 × 10⁻⁵. Its pKa = −log₁₀(1.8 × 10⁻⁵) = 4.74. Going the other way, an acid with pKa 3.0 has Ka = 10⁻³ = 1.0 × 10⁻³, so it is about 55 times stronger than acetic acid.
Frequently asked questions
Does a higher pKa mean a stronger or weaker acid?
A higher pKa means a weaker acid. pKa is the negative log of Ka, so as Ka gets smaller (weaker acid) the pKa gets larger.
What is the difference of one pKa unit?
One pKa unit is a factor of ten in Ka. An acid with pKa 3 is ten times stronger than one with pKa 4, and a hundred times stronger than one with pKa 5.
How does this relate to a buffer?
A buffer works best near the pKa of its weak acid, because that is where equal amounts of acid and conjugate base coexist. The Henderson–Hasselbalch equation uses pKa directly.
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This calculator is for educational purposes. Double-check important results before acting on them.