Dissociation Constant Tool Calculator

Calculator

Choose a model, enter inputs, then submit.

Calculation Mode

All concentrations are in mol/L (M).

Initial concentration, C0 (M)

For monoprotic weak acids: HA ⇌ H⁺ + A^-.

Measured pH

Uses [H⁺] = 10^-pH.

Equilibrium [H⁺] (M)

Enter measured hydrogen ion concentration.

Equilibrium [A^-] (M)

Often ~ [H⁺] for monoprotic acids.

Equilibrium [HA] (M)

Undissociated acid at equilibrium.

Initial concentration, C0 (M)

For weak bases: B + H2O ⇌ BH⁺ + OH^-.

Measured pH

Uses pOH = 14 − pH, [OH^-] = 10^-pOH.

Equilibrium [OH^-] (M)

Enter measured hydroxide concentration.

Equilibrium [BH⁺] (M)

Conjugate acid concentration at equilibrium.

Equilibrium [B] (M)

Unprotonated base remaining at equilibrium.

Conversion

Assumes Kw = 1.0×10^-14 at 25°C.

Value

Scientific notation is supported.

Reset

Example Data Table

Scenario	Inputs	Output	Interpretation
Weak acid (from pH)	C0 = 0.100 M, pH = 2.87	Ka ≈ 1.8 × 10^-5, pKa ≈ 4.74	Typical weak acid strength range.
Weak base (from pH)	C0 = 0.050 M, pH = 11.12	Kb ≈ 1.6 × 10^-5, pKb ≈ 4.80	Moderate base; partial protonation.
Conversion	pKa = 3.20	Ka = 6.31 × 10^-4	Lower pKa implies stronger acid.

Examples are illustrative; your measurements may differ.

Formula Used

Weak acid

For HA ⇌ H⁺ + A⁻:

Ka = ([H⁺][A⁻]) / [HA]
pKa = −log10(Ka)
From pH mode uses [H⁺] = 10^−pH and [HA] = C0 − [H⁺].

Weak base

For B + H2O ⇌ BH⁺ + OH⁻:

Kb = ([BH⁺][OH⁻]) / [B]
pKb = −log10(Kb)
From pH mode uses pOH = 14 − pH and [OH⁻] = 10^−pOH.

Linking conjugates

At 25°C, Kw = [H⁺][OH⁻] = 1.0 × 10^−14. For a conjugate acid–base pair: Ka × Kb = Kw.

How to Use This Calculator

Select a mode that matches your data source.
Enter concentrations in mol/L, or enter pH when required.
Use equilibrium mode if you already measured species concentrations.
Click Submit to display Ka/Kb and pKa/pKb.
Use Download CSV or Download PDF for reports.
If you see warnings, revise assumptions or switch modes.

Note: pH-based modes assume a simple monoprotic system and dilute aqueous behavior. Polyprotic acids, buffers, and high ionic strength require more advanced treatment.

Dissociation constants and chemical meaning

Ka and Kb quantify the position of an acid or base equilibrium in water. In this calculator, Ka is computed from measured [H+], [A−], and [HA], while pKa expresses the same strength on a logarithmic scale for easy comparison across orders of magnitude. Typical weak acids span Ka≈10^−2 to 10^−10, and strong acids effectively behave as fully dissociated under dilute conditions.

Data quality and measurement inputs

Reliable Ka or Kb depends on consistent concentration units, calibrated pH electrodes, and temperature control. A 0.01 pH shift changes [H+] by about 2.3%, which propagates into Ka when pH-derived modes assume [A−]≈[H+]. Enter values in mol/L and keep significant figures aligned with your lab data. For titration-derived concentrations, use equilibrium values after dilution corrections, not stock molarity.

Using pH-based estimation responsibly

The pH modes treat a monoprotic system where dissociation produces equal amounts of conjugate and hydrogen or hydroxide ions. For weak acids, x=10^−pH and Ka=x^2/(C0−x). For weak bases, pOH=14−pH, x=10^−pOH, and Kb=x^2/(C0−x). If x approaches C0, switch to equilibrium mode. As a quick check, percent dissociation is 100·x/C0; values above ~10% often warrant activity corrections.

Conjugate relationships and conversions

For conjugate pairs at 25°C, Ka×Kb=Kw with Kw≈1.0×10^−14. Converting between Ka, Kb, pKa, and pKb helps compare acids and bases on a common scale. Smaller pKa indicates a stronger acid, while smaller pKb indicates a stronger base, assuming identical conditions. Example: pKa 4.74 implies Ka≈1.82×10^−5, matching many carboxylic acids in water.

Interpreting results for decisions

Use Ka or pKa to rank acids for buffer design, reaction selectivity, and salt formation. Use Kb or pKb to compare bases for protonation control and extraction planning. Exported CSV and PDF outputs support documentation, peer review, and repeatability, especially when you record temperature, ionic strength notes, and the chosen calculation mode. When comparing literature values, ensure the same temperature and solvent system, since Ka can shift measurably across conditions. For routine reporting, include the raw inputs, the calculated Ka or Kb in scientific notation, and the corresponding p value rounded to three to six decimals. If results look inconsistent, recheck electrode slope, contamination, and equilibrium time before rerunning in your lab notebook.

FAQs

1) Can I use this for polyprotic acids?

It is designed for single-step (monoprotic) behavior. For polyprotic systems, each dissociation has its own Ka, and pH depends on multiple equilibria. Use equilibrium concentrations per step or specialized speciation software.

2) Why does the tool warn when [H+] is close to C0?

When x≈C0, the assumption [HA]=C0−x becomes unstable and small measurement errors dominate. In such cases, use measured equilibrium concentrations or a different experimental setup with higher C0.

3) What temperature does the Ka↔Kb conversion assume?

Conversions use Kw≈1.0×10^−14, which corresponds to 25°C in dilute water. If temperature differs, Kw changes and Ka×Kb will not equal 1.0×10^−14. Use a temperature-adjusted Kw for best accuracy.

4) Should I enter activities instead of concentrations?

If you have activity coefficients, activities can improve accuracy in higher ionic strength solutions. Most instructional and dilute-lab use cases rely on concentrations. Keep conditions consistent when comparing values across datasets.

5) How many decimals should I report for pKa or pKb?

Match your measurement precision. With typical pH uncertainty of ±0.01–0.02, reporting 2–3 decimals is often reasonable. More decimals may look precise but can be misleading without tighter experimental control.

6) What does a very small Ka or Kb mean?

A smaller constant indicates the equilibrium favors the undissociated form: weaker acid (small Ka) or weaker base (small Kb). Log forms (pKa, pKb) make these comparisons easier across large ranges.