Calculate the acid dissociation constant (Ka) of a weak acid from its measured pH and initial molar concentration.
Equilibrium Data
M
Calculation Results
Enter the solution pH and initial molar concentration to calculate Ka
About this calculator
Overview
The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for the chemical reaction of dissociation.
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Pro Tips
Calculated [H⁺] is derived from the pH: [H⁺] = 10⁻ᵖᴴ.
The formula used is Ka = [H⁺]² / (C - [H⁺]), assuming a single dissociation step (HA ⇌ H⁺ + A⁻).
For very weak acids or very dilute solutions, the C - [H⁺] term is approximately equal to C.
pKa is more convenient than Ka for comparing acid strengths across many orders of magnitude.
The Henderson-Hasselbalch equation relates pH, pKa, and the ratio of conjugate base to acid.
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Fun Facts
"A higher Ka value means the acid is stronger and dissociates more completely in water."
"Strength is also often expressed as pKa, where pKa = -log₁₀(Ka). Stronger acids have lower pKa values."
"Polyprotic acids, like sulfuric acid, have multiple dissociation constants (Ka₁, Ka₂, etc.)."
"The pKa scale ranges from -10 to 20, with most common acids falling between 0 and 14."
"Acids with pKa < 0 are considered strong acids and completely dissociate in water."
"The difference between Ka and Kb for conjugate acid-base pairs is related by Kw (Ka × Kb = Kw)."
"Temperature affects Ka values, with most acids showing increased dissociation at higher temperatures."
Formula: Ka = [H⁺]² / (C - [H⁺])
Acid Strength Scale
Common acids arranged by increasing pKa (decreasing acid strength)
