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pKa Calculator

Calculate pKa from Ka or Ka from pKa instantly. Convert between acid dissociation constant (Ka) and its negative logarithm (pKa) with step-by-step solutions. Free online chemistry tool.

Choose whether to convert Ka to pKa or pKa to Ka

Enter the Ka value (for Ka → pKa) or the pKa value (for pKa → Ka)

What is a pKa Calculator?

The pKa calculator is a fundamental tool for chemistry students, researchers, and professionals working with acids, bases, and buffer solutions. The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It represents the equilibrium constant for the dissociation of an acid (HA) into a proton (H⁺) and its conjugate base (A⁻) in aqueous solution. Because Ka values can span many orders of magnitude — from extremely strong acids like hydrochloric acid (Ka ≈ 10⁶) to very weak acids like water (Ka ≈ 1.8 × 10⁻¹⁶) — chemists commonly use the pKa scale instead. The pKa is simply the negative base-10 logarithm of Ka, making it much easier to compare acid strengths on a compact numerical scale. A lower pKa indicates a stronger acid (more complete dissociation), while a higher pKa indicates a weaker acid. For instance, acetic acid has a Ka of 1.8 × 10⁻⁵ and a pKa of 4.74, meaning it is a weak acid that only partially dissociates in water. Understanding pKa is essential for predicting acid-base reaction outcomes, designing buffer systems, interpreting titration curves, and studying biochemical processes such as enzyme activity and drug pharmacokinetics. This free online pKa calculator allows you to instantly convert between Ka and pKa values with detailed step-by-step explanations.

Formula

pKa=−log⁡10(Ka)andKa=10−pKapK_a = -\log_{10}(K_a) \quad \text{and} \quad K_a = 10^{-pK_a}

Where Ka is the acid dissociation constant and pKa is its negative base-10 logarithm. These two formulas are inverses of each other and allow seamless conversion between the linear Ka scale and the logarithmic pKa scale.

How to Calculate

  1. 1

    Select the conversion mode: Ka → pKa if you have the acid dissociation constant, or pKa → Ka if you have the pKa value.

  2. 2

    Enter the known value in the input field. For Ka values, you can use scientific notation (e.g., 1.8e-5 for 1.8 × 10⁻⁵).

  3. 3

    Click Calculate to get the result with a step-by-step breakdown of the conversion.

  4. 4

    Review the steps to understand how the logarithmic or exponential conversion was applied.

  5. 5

    Use the results for further calculations such as buffer design, pH computation, or acid strength comparison.

Worked Examples

Convert Ka of Acetic Acid to pKa

Input: Ka = 1.8 × 10⁻⁵

  1. Formula: pKa = -log₁₀(Ka)
  2. Substitution: pKa = -log₁₀(1.8 × 10⁻⁵)
  3. Evaluate: log₁₀(1.8 × 10⁻⁵) = log₁₀(1.8) + log₁₀(10⁻⁵) = 0.2553 + (-5) = -4.7447
  4. Result: pKa = -(-4.7447) = 4.74

Result: pKa = 4.74

Convert pKa of Formic Acid to Ka

Input: pKa = 3.75

  1. Formula: Ka = 10^(-pKa)
  2. Substitution: Ka = 10^(-3.75)
  3. Evaluate: 10^(-3.75) = 1.778 × 10⁻⁴
  4. Result: Ka = 1.778 × 10⁻⁴

Result: Ka = 1.778 × 10⁻⁴

Convert Ka of Hydrofluoric Acid to pKa

Input: Ka = 6.8 × 10⁻⁴

  1. Formula: pKa = -log₁₀(Ka)
  2. Substitution: pKa = -log₁₀(6.8 × 10⁻⁴)
  3. Evaluate: log₁₀(6.8 × 10⁻⁴) = log₁₀(6.8) + log₁₀(10⁻⁴) = 0.8325 + (-4) = -3.1675
  4. Result: pKa = -(-3.1675) = 3.17

Result: pKa = 3.17

Frequently Asked Questions

Ka (acid dissociation constant) is the equilibrium constant for the dissociation of an acid in water, expressed as a ratio of product concentrations to reactant concentrations. pKa is the negative base-10 logarithm of Ka (pKa = -log₁₀Ka). They convey the same information on different scales: Ka uses very small or very large numbers, while pKa compresses them into a more convenient range. A smaller Ka (or larger pKa) means a weaker acid.
A low pKa value indicates a strong acid that dissociates more completely in water. For example, hydrochloric acid has a pKa of about -7, meaning it almost fully dissociates. In contrast, acetic acid has a pKa of 4.74 and only partially dissociates. Generally, acids with pKa values below 0 are considered strong acids, while those above 0 are weak acids.
Yes, pKa can be negative. A negative pKa occurs when Ka is greater than 1, which is the case for strong acids. For example, hydrochloric acid (HCl) has a Ka ≈ 10⁷ and a pKa ≈ -7. Negative pKa values indicate that the acid is very strong and essentially dissociates completely in aqueous solution.
The Henderson-Hasselbalch equation directly relates pKa to pH: pH = pKa + log₁₀([A⁻]/[HA]). At the half-equivalence point of a titration, when [A⁻] = [HA], pH equals pKa. This relationship is critical for designing buffer solutions, which resist pH changes most effectively when the desired pH is close to the pKa of the buffering acid.
The pKa of water is approximately 15.7 at 25°C. This corresponds to a Ka of about 1.8 × 10⁻¹⁶. Water is an extremely weak acid, meaning it has very little tendency to donate a proton to another water molecule. This very high pKa value is why pure water is essentially neutral.

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