Pka Calculator - Ka, pKa, and Weak Acid pH

A pKa calculator converts between the acid dissociation constant Ka and its logarithmic form pKa, then estimates the pH of a monoprotic weak acid solution from the concentration and Ka in a single pass. It returns pKa, Ka, the weak-acid pH, and the implied hydrogen ion concentration so you can move between table values, equilibrium calculations, and bench-scale preparation without retyping the formulas.

• • General chemistry homework: Convert Ka to pKa (or vice versa) for textbook problems on acetic acid, formic acid, and ammonium.
• • Biochemistry buffer prep: Look up the Ka that pairs with a published pKa before weighing out a phosphate buffer near pH 7.4.
• • Pharmacology and drug pKa lookup: Translate the pKa in a drug monograph into Ka for Henderson-Hasselbalch absorption estimates.
• • Analytical method development: Check whether the pKa of an analyte matches the mobile-phase pH of an HPLC method.

Most chemistry references quote pKa because it is easier to compare numbers like 4.76 (acetic acid) and 9.25 (ammonium) than the original Ka values of 1.74e-5 and 5.62e-10, and the calculator uses the same pKw = 14 baseline as the other chemistry calculators on the site.

Once you have Ka and a concentration and want the pH value, pH & pOH Calculator converts back and forth between pH, pOH, and hydrogen ion concentration so the two tools can be used together.

The calculator reads the mode selector, picks whichever of Ka or pKa you supplied, computes the other value by applying log10 or 10^(-pKa), then estimates the weak-acid pH from the analytical concentration using the standard [H+] = sqrt(Ka * C) shortcut.

• Ka: Acid dissociation constant for the monoprotic weak acid HA -> H+ + A-. Either supplied directly or recovered from the entered pKa.
• pKa: Negative base-10 logarithm of Ka. Either supplied directly or computed from Ka via pKa = -log10(Ka).
• C (mol/L): Molar concentration of the weak acid in mol/L. Combined with Ka to estimate the solution pH via [H+] = sqrt(Ka * C).
• [H+] (mol/L): Equilibrium hydrogen ion concentration implied by the calculated pH; returned alongside pH so you can spot-check the math.

When you flip the mode selector to pKa the calculator still derives Ka first, because every downstream quantity (pKa, the weak-acid pH, and [H+]) needs Ka in the same units. The conversion is purely algebraic, so the value you see for pKa matches the value a chemistry textbook prints for the same acid.

The weak-acid pH branch assumes Ka is much smaller than the concentration so dissociation does not appreciably change [HA]; when Ka approaches or exceeds C use the quadratic solution Ka = [H+]^2 / (C - [H+]) instead.

According to Chemistry LibreTexts, pKa uses the same negative-log convention as pH and pOH, so pKa = -log10(Ka) and the conversion works in either direction.

According to Chemistry LibreTexts, weak acids only partially dissociate, so [H+] collapses to sqrt(Ka * C) when Ka is much smaller than the analytical concentration.

The Ka you just computed is exactly the value Henderson-Hasselbalch needs, so pair the pKa output with Buffer pH Calculator to predict the pH of an HA/A- buffer at any acid to base ratio.

Four ideas connect every output the pKa calculator returns, from the meaning of pKa to the limits of the weak-acid approximation.

Together these four ideas cover every number the calculator shows; the most common cause of a printed Ka or pKa not matching a reference table is mixing up which dissociation of a polyprotic acid you are looking at (for example the pKa 7.20 of dihydrogen phosphate vs the pKa 12.4 of hydrogen phosphate).

When you have the molecular formula of the acid and want the molar mass that pairs with the molarity entered above, Mole & Molar Mass Calculator returns the grams per mole needed to convert moles into a mass to weigh.

Pick which value you already know, enter it together with the analytical concentration, and the calculator returns pKa, Ka, the weak-acid pH, and the implied hydrogen ion concentration.

1. 1 Choose the primary input: Select I have Ka if you are working from a textbook value or analytical chemistry reference, or I have pKa if you are working from a buffer table or drug monograph.
2. 2 Enter Ka or pKa: Type the Ka in scientific notation (for example 1.74e-5 for acetic acid) or the pKa as a plain number (4.76 for acetic acid). Leave the other field blank; the calculator will fill it in.
3. 3 Enter the weak-acid concentration: Type the molarity of the monoprotic weak acid in mol/L; values from 1e-6 to 10 mol/L are accepted.
4. 4 Read pKa and Ka: Note the pKa and Ka the calculator returns; the two values are internally consistent because they are computed from the same input.
5. 5 Read the weak-acid pH: Use the weak-acid pH as a quick estimate of how acidic the solution would be at the entered concentration.
6. 6 Adjust if needed: If the weak-acid pH looks wrong, double-check the concentration and the dissociation you are targeting.

For 0.10 M acetic acid, switch to Ka mode and enter Ka = 1.74e-5 with concentration = 0.10 mol/L. The calculator returns pKa = 4.76, Ka = 1.74e-5, weak-acid pH = 2.88, and [H+] = 1.32e-3 mol/L. To estimate the same solution at 0.001 mol/L instead, leave Ka at 1.74e-5 and change the concentration; the calculator then returns weak-acid pH = 3.88.

To turn the molarity you entered into a mass to weigh on the bench, Grams to Moles Calculator converts grams and molar mass into moles first so the buffer prep lines up with the weak-acid pH estimate.

A pKa calculator compresses the table-lookup plus the weak-acid approximation into one screen.

• • Bidirectional conversion: Move from Ka to pKa or from pKa to Ka in one step, without writing -log10 by hand.
• • Built-in weak-acid pH: Combine the Ka with a concentration to estimate the pH of a monoprotic weak acid solution.
• • Consistent chemistry reference: Use the same pKw 14 baseline as the buffer pH and pH and pOH calculators on the site.
• • Quick buffer prep sanity check: Cross-check the Ka from a published pKa against the buffer pH from Henderson-Hasselbalch before mixing reagents.
• • Helpful for pharmacology and biology: Translate drug-monograph pKa values into Ka for Henderson-Hasselbalch absorption or solubility estimates.

If you also need the mass of weak acid to weigh out, pair the pKa calculator with the grams-to-moles tool to convert the molarity into grams of reagent.

Five variables drive the values the pKa calculator returns, and the same variables explain when the weak-acid approximation is reliable and when it is not.

• • The weak-acid pH branch uses the approximation [H+] = sqrt(Ka * C), which ignores water autoionization; for solutions below about 1e-5 M the contribution from water can shift the pH noticeably.
• • Polyprotic acids such as phosphate, carbonate, citrate, and amino acids contribute more than one dissociation step.
• • The calculator does not correct for activity coefficients, so values in high-salt backgrounds need an activity correction.

These factors are the most common reasons a calculated pH differs from a calibrated pH electrode reading. The calculator gives the analytical-concentration prediction that textbook problems expect.

According to Chemistry LibreTexts, common laboratory pKa values include 4.76 for acetic acid, 3.75 for formic acid, and 7.20 for dihydrogen phosphate.

If you want to know how much strong acid or base the same weak acid solution can absorb before the pH drifts by one unit, Buffer Capacity Calculator runs the Van Slyke equation on the Ka and total concentration you just computed.