Enter your Calculation Method, then provide either a Ka Value or your pH, Conjugate Base Concentration [A⁻], and Acid Concentration [HA] to get your pKa Value — plus a handy Acid Strength Classification, the Calculated Ka Value, and the [A⁻]/[HA] Ratio thrown in alongside it.
pKa Value
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Acid Strength Classification
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Calculated Ka Value
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[A⁻]/[HA] Ratio
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pKa is the negative logarithm of the acid dissociation constant (Ka). It determines how weak or strong an acid is - the lower the pKa, the stronger the acid and the more readily it donates protons.
Use the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). Rearranging: pKa = pH - log([A⁻]/[HA]), where [A⁻] is the conjugate base concentration and [HA] is the acid concentration.
pKa is simply the negative logarithm of Ka: pKa = -log₁₀(Ka). For example, if Ka = 1.8 × 10⁻⁵, then pKa = -log(1.8 × 10⁻⁵) = 4.74.
Ka is the acid dissociation constant showing the extent of acid ionization, while pKa is the negative logarithm of Ka. pKa values are more convenient as they're typically positive numbers, with strong acids having low pKa values and weak acids having high pKa values.
No, pH measures the acidity of a solution (concentration of H⁺ ions), while pKa is an intrinsic property of the acid itself. However, they're related through the Henderson-Hasselbalch equation.
Strong acids have pKa values less than 0, weak acids have pKa values between 0-14, and very weak acids have pKa values greater than 14. The lower the pKa, the more readily the acid donates protons.
The Henderson-Hasselbalch equation is most accurate for pH values within ±1 unit of the pKa and when the acid concentration is much greater than the H⁺ or OH⁻ concentrations.
This calculator works for monoprotic acids or individual ionization steps of polyprotic acids. For polyprotic acids, each ionizable proton has its own pKa value that should be calculated separately.