pKa and pKb Calculator
Convert freely between Ka, pKa, Kb and pKb.
Work out pKa and pKb. Convert freely between Ka, pKa, Kb and pKb. Written for the problem set you are stuck on.
Scientific notation works. Acetic acid Ka is 1.8e-5, ammonia Kb is 1.8e-5.
pKa
4.745
Moderately strong weak acid
pKa + pKb = 14 holds for a CONJUGATE PAIR at 25 °C, because Ka·Kb = Kw. It is not a property of a single substance, and pairing an unrelated acid with an unrelated base makes it meaningless. A lower pKa is a stronger acid, and each unit is a factor of ten. The relationship is inverse, so a strong acid necessarily has a weak conjugate base — chloride, from hydrochloric acid, is so weak a base that it does nothing in water at all. The 14 comes from Kw at 25 °C. At 37 °C water's ion product rises and neutral pH is 6.81, so the sum is 13.62 — which matters in physiology, though rarely in a teaching lab.
How the pKa and pKb Calculator works
Give any one of Ka, pKa, Kb or pKb and this returns the other three for the conjugate pair, along with a reading of the acid's strength. The pKa + pKb = 14 relationship holds for a conjugate pair at 25 °C, and the page is explicit that it is not a property of a single substance.
Also known as: ka to pka converter · pka to ka calculator · acid dissociation constant calculator · pkb calculator
Frequently asked questions
How do I convert Ka to pKa?
pKa = −log₁₀(Ka). Acetic acid's Ka of 1.8 × 10⁻⁵ gives a pKa of 4.74. Going back, Ka = 10^(−pKa).
Does pKa + pKb always equal 14?
Only for a conjugate acid-base pair, and only at 25 °C, because it comes from Ka·Kb = Kw. Pairing an unrelated acid with an unrelated base makes the sum meaningless.
Does a lower pKa mean a stronger acid?
Yes, and each unit is a factor of ten in acidity. A pKa below zero indicates a strong acid that is effectively fully dissociated in water.
Why does a strong acid have a weak conjugate base?
Because Ka and Kb are inversely related through Kw. Hydrochloric acid is strong, so chloride is such a weak base that it does nothing measurable in water at all.
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