pKa Calculator
Solve for pKa or Ka directly using pKa = -log10(Ka), or back out pKa from a measured pH and the conjugate base to acid ratio. This calculator solves for pKa/Ka, not pH; for buffer pH from a known pKa, use the Buffer pH Calculator instead.
🧪 What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant Ka, pKa = −log₁₀(Ka). Ka itself measures how completely a weak acid dissociates into ions in water: a larger Ka means more dissociation and a stronger acid. Because Ka values for weak acids span an enormous range, from around 10⁻² down to 10⁻¹⁴ or smaller, chemists use the logarithmic pKa scale instead, the same way pH is used instead of raw hydrogen ion concentration.
This calculator solves specifically for pKa or Ka, not for the pH of a buffer. In direct mode, you supply either Ka or pKa and it converts to the other using pKa = −log₁₀(Ka) or the inverse Ka = 10−pKa. In Henderson-Hasselbalch mode, you supply a measured pH and the concentrations of a conjugate acid-base pair, and the calculator rearranges the Henderson-Hasselbalch equation to solve for the unknown pKa: pKa = pH − log([A⁻] / [HA]).
This is the reverse of the site's Buffer pH Calculator, which assumes pKa is already known and solves for pH. Use this pKa Calculator whenever pKa or Ka is the unknown you need to find, for example when determining an unknown acid's identity from a titration curve, or when converting a reported Ka value from a reference table into the more commonly quoted pKa form.
Because pKa is logarithmic, a difference of just 1 pKa unit means a tenfold difference in Ka and therefore in acid strength. This calculator also classifies the resulting Ka into a rough strong, weak, or very weak category and plots log₁₀(Ka) against pKa so you can see where your acid sits on the overall acidity scale.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Acetic Acid, Ka Known
Ka = 1.8 × 10⁻⁵ mol/L, direct mode
Example 2 — Known pKa, Find Ka
Example 3 — Finding pKa from a Measured pH
pH = 5.0, [A⁻] = 0.2 mol/L, [HA] = 0.1 mol/L, Henderson-Hasselbalch mode
❓ Frequently Asked Questions
🔗 Related Calculators
What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant Ka: pKa = -log10(Ka). It compresses the very wide range of Ka values, which can span many orders of magnitude, into a more convenient linear scale, similar to how pH compresses hydrogen ion concentration.
How do you calculate pKa from Ka?
Take the negative base-10 logarithm of Ka: pKa = -log10(Ka). For example, acetic acid has Ka = 1.8 x 10^-5, so pKa = -log10(1.8 x 10^-5) = 4.74. A smaller Ka gives a larger pKa, meaning the acid dissociates less and is weaker.
How do you calculate Ka from pKa?
Raise 10 to the power of the negative pKa: Ka = 10^(-pKa). For example, a pKa of 4.76 gives Ka = 10^(-4.76) = 1.74 x 10^-5. This is the inverse operation of finding pKa from Ka.
How does this calculator differ from the Buffer pH Calculator?
The pKa Calculator solves for pKa or Ka, either directly by conversion or from a measured pH using Henderson-Hasselbalch rearranged as pKa = pH - log([A-]/[HA]). The Buffer pH Calculator instead assumes you already know pKa and solves the opposite direction, pH = pKa + log([A-]/[HA]), to find a buffer's pH. Use the pKa Calculator when pKa or Ka is the unknown.
How do you find pKa from a measured pH?
Rearrange the Henderson-Hasselbalch equation to solve for pKa: pKa = pH - log([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] is the weak acid concentration. This is exactly how pKa values are often determined experimentally, by measuring the pH of a solution with known acid and conjugate base concentrations.
What does a low pKa mean?
A low pKa means a high Ka, since pKa = -log10(Ka), and a high Ka means the acid dissociates almost completely in water, releasing protons readily. Strong acids like hydrochloric acid have pKa values well below 0 (or are considered to dissociate essentially completely), while weak acids like acetic acid have pKa around 4 to 5.
What is the relationship between pKa and acid strength?
Acid strength and pKa are inversely related: the smaller the pKa, the stronger the acid, because a smaller pKa corresponds to a larger Ka and more complete dissociation into ions. Acids with pKa below about 0 are typically classified as strong, and acids with pKa between roughly 2 and 12 are typically classified as weak.
Why is pKa used instead of Ka directly?
Ka values for weak acids often span many orders of magnitude, from around 10^-2 down to 10^-14 or smaller, which is awkward to compare and graph directly. Taking the negative logarithm compresses this huge range into a more manageable scale, usually between about -10 and 50, similar to how pH is used instead of hydrogen ion concentration.
Is pKa the same for every temperature?
No. Ka, and therefore pKa, is an equilibrium constant and depends on temperature, since the dissociation equilibrium shifts with temperature like any chemical equilibrium. Reported pKa values are almost always specified at 25 degrees C unless stated otherwise, and can shift noticeably at body temperature or other conditions.
What is the half-equivalence point in an acid-base titration?
The half-equivalence point is where exactly half of a weak acid has been converted to its conjugate base during a titration, making [A-] equal to [HA]. At this point the Henderson-Hasselbalch equation gives pH = pKa + log(1) = pKa, so measuring the pH at half-equivalence directly gives the acid's pKa.
Can pKa be negative?
Yes. Very strong acids have Ka values much greater than 1, and taking -log10 of a number greater than 1 gives a negative result. For example, hydrochloric acid is often quoted with a pKa around -6 to -7, reflecting its essentially complete dissociation in water.