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Weak Acid pH Calculator
This is a calculator to find values of a weak acid dissociation reaction. All concentrations are measured in M (mol/liter). Values are found using the equilibrium constant formula, where the product of the concentration of products to the power of their coefficients is divided by the product of the concentration of reactants to the power of their coefficients. pKa is found by finding ten to the power of the equilibrium constant. Conversely, the equilibrium constant is used to find the pKa value by taking the negative log of the equilibrium constant value entered. The pH value is found by taking the negative log of weak acid product. This calculator can be used to fill in values of an ICE table, find the equilibrium constant of the reaction and equilibrium concentration of reactant, as well as the resulting pH of the reaction. It can also be used to convert between the equilibrium constant and pKa values.
Select which value to solve for:
HA
A-
+
H+
I (nitial)
C (hange)
E (quilibrium)
ParameterValue
Volume of reaction (in litre, L)
Acid dissociation constant ( Ka )
pKa
pHN/A

Additional Information
Although calculating the pH of a monoprotic acid or base is quite simple, doing so for a polyprotic compound can often be much more complex. Polyprotic acids and bases have multiple ionizable hydrogen ions or hydroxide ions per molecule, and undergo multiple dissociation steps as a result. Furthermore, they may have different dissociation constants for each ionization step, which must be accounted for when calculating pH. In order to provide an accurate pH value for polyprotic compounds, this tool takes into account each of the dissociation constants and calculates the change in H+ or OH- concentration from each dissociation step. The tool also utilizes a univariate polynomial equation to find the exact concentration change in each step, rather than assuming that any of the dissociation constants are negligible.
It should be noted that while the pH calculation performed by this tool takes into account each dissociation step for polyprotic acids and bases, the ICE table only depicts the first dissociation step. In order to get a visual representation of the change in H+ or OH- concentration from each step for a polyprotic compound, multiple ICE tables must be computed using updated initial concentration values and the corresponding Ka value for each step.

Feedback
Have a question or a feature request about this tool? Feel free to reach out to us and let us know! We're always looking for ways to improve!

References
This online resource may be cited as follows
MLA
"Quest CalculateWeak Acid pH Calculator." AAT Bioquest, Inc.23 Sep2025https://www.aatbio.com/tools/weak-acid-conjugate-base-ph-pka-calculator.
APA
AAT Bioquest, Inc. (2025September 23). Quest CalculateWeak Acid pH Calculator. AAT Bioquest. https://www.aatbio.com/tools/weak-acid-conjugate-base-ph-pka-calculator.
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