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pKa Calculator

Chemistry

Calculate pKa from acid dissociation constant Ka, or from pH and the concentrations of acid and conjugate base. Includes pKb and Kb outputs.

0

pKa

5.745
pKb
8.255
Kb (conjugate base)
0
Acid Strength
Weak acid

This calculator computes your pKa, pKb, Kb (conjugate base), Acid Strength from the values you enter.

Inputs
Acid Dissociation Constant (Ka)
Outputs
pKapKbKb (conjugate base)Acid Strength

What is a pKa?

The pKa Calculator converts the acid dissociation constant (Ka) of a weak acid to its pKa value using the formula pKa = −log₁₀(Ka). It also returns pKb and Kb for the conjugate base, and classifies the acid by strength — giving a complete picture of the acid-base pair from a single Ka input.

pKa is the most compact and practical way to express the strength of a weak acid. The underlying Ka values for common weak acids range from 10⁻² (moderately strong) to 10⁻¹⁴ (extremely weak), a twelve-order-of-magnitude span that is impossible to compare at a glance. The pKa scale compresses this into a 2–14 range: the lower the pKa, the stronger the weak acid. Acetic acid (pKa 4.74) is stronger than ammonium ion (pKa 9.25) — it dissociates more at the same concentration, giving a higher [H⁺] and lower pH.

The relationship pKa + pKb = 14 (at 25°C) links every weak acid to its conjugate base. If you know the pKa of acetic acid (4.74), you immediately know the pKb of acetate ion (9.26) and can calculate how much acetate will hydrolyse in water. This connection between Ka and Kb is fundamental to understanding why mixing a weak acid with its conjugate base creates a buffer — the forward and reverse reactions are both characterised by equilibrium constants that are precise reciprocals of each other.

pKa is the key input to the Henderson-Hasselbalch Calculator and the Buffer pH Calculator. When designing a buffer for a biochemistry experiment, choosing an acid whose pKa is within one unit of the target pH maximises buffer capacity. For enzyme activity assays, drug formulation buffers, and HPLC mobile phase preparation, pKa selection directly determines how well the solution resists pH change.

In the Indian chemistry curriculum, Ka and pKa are introduced in NCERT Class 11 Chapter 7 and are tested in JEE Main, Advanced, and NEET in problems ranging from simple pKa conversions to multi-step buffer and titration calculations.

How to use this pKa calculator

  1. Find Ka for your acid — look up the acid dissociation constant in a reference table (e.g., NCERT appendix, CRC Handbook, or a standard analytical chemistry textbook). Common values: acetic acid Ka = 1.8 × 10⁻⁵, carbonic acid Ka₁ = 4.3 × 10⁻⁷, ammonium ion Ka = 5.6 × 10⁻¹⁰.
  2. Enter Acid Dissociation Constant (Ka) — type Ka into the Acid Dissociation Constant (Ka) field. Enter small values as decimals (e.g., 0.0000018 for 1.8 × 10⁻⁵) or in scientific notation if supported. The field accepts values as small as 10⁻²⁰.
  3. Read pKa — the highlighted output gives pKa to four decimal places. For acetic acid (Ka = 1.8 × 10⁻⁵), the result is pKa = 4.7447.
  4. Read pKb — the conjugate base's pKb = 14 − pKa. Use this to assess how basic the conjugate salt will be in water.
  5. Read Kb — the numeric base dissociation constant for the conjugate base. Use this in Kb equilibrium expressions.
  6. Check Acid Strength — the classification label confirms the acid's strength category. For buffer design, take the pKa value to the Buffer pH Calculator and check whether it falls within ±1 pH unit of your target buffer pH.

Formula & Methodology

pKa formula:

> pKa = −log₁₀(Ka)

Inverse:

> Ka = 10^(−pKa)

Conjugate base pKb:

> pKb = 14 − pKa  (at 25°C, from pKa + pKb = pKw = 14)

Conjugate base Kb:

> Kb = 10^(−pKb) = 10^(−(14 − pKa)) = 10^(pKa − 14)

Relationship verification:

> Ka × Kb = Kw = 10⁻¹⁴ mol²/L² (at 25°C)

Worked example 1 — Acetic acid:

- Ka of CH₃COOH = 1.8 × 10⁻⁵
- pKa = −log₁₀(1.8 × 10⁻⁵) = −[log(1.8) + log(10⁻⁵)] = −[0.2553 − 5] = 4.745
- pKb (acetate) = 14 − 4.745 = 9.255
- Kb = 10⁻⁹˙²⁵⁵ = 5.56 × 10⁻¹⁰
- Verification: Ka × Kb = 1.8 × 10⁻⁵ × 5.56 × 10⁻¹⁰ = 1.0 × 10⁻¹⁴ ✓

Worked example 2 — Ammonium ion (JEE context):

- Ka of NH₄⁺ = 5.56 × 10⁻¹⁰
- pKa = −log₁₀(5.56 × 10⁻¹⁰) = 9.255
- pKb (NH₃) = 14 − 9.255 = 4.745
- This confirms the well-known relationship: pKa(NH₄⁺) + pKb(NH₃) = 9.255 + 4.745 = 14 ✓
- NH₃ is a stronger base (lower pKb) than acetate ion (pKb 9.255)

pKa reference table for common acids:

| Acid | Ka | pKa |
|---|---|---|
| Oxalic acid (1st) | 5.6 × 10⁻² | 1.25 |
| Phosphoric acid (1st) | 7.5 × 10⁻³ | 2.15 |
| Acetic acid | 1.8 × 10⁻⁵ | 4.74 |
| Carbonic acid (1st) | 4.3 × 10⁻⁷ | 6.37 |
| Dihydrogen phosphate (2nd) | 6.2 × 10⁻⁸ | 7.21 |
| Ammonium ion | 5.6 × 10⁻¹⁰ | 9.25 |
| Bicarbonate (2nd Ka) | 4.7 × 10⁻¹¹ | 10.33 |

Use the Hydrogen Ion Concentration Calculator to find [H⁺] from pH once pKa and buffer concentrations are known.

Frequently Asked Questions

pKa is the negative base-10 logarithm of the acid dissociation constant Ka: pKa = −log₁₀(Ka). It measures the strength of a weak acid — the lower the pKa, the stronger the acid (it donates protons more readily at a given concentration). For example, acetic acid has pKa = 4.76, while hydrofluoric acid has pKa = 3.17, making HF a stronger acid than acetic acid despite both being classified as weak acids.
Ka is the equilibrium constant for the dissociation of a weak acid (HA) in water: HA ⇌ H⁺ + A⁻. The expression is Ka = [H⁺][A⁻] / [HA], where square brackets denote equilibrium concentrations in mol/L. A large Ka means the acid dissociates extensively (stronger acid, lower pKa); a small Ka means it dissociates little (weaker acid, higher pKa). Strong acids like HCl have Ka values in the thousands and are not described using Ka at all.
pKa = −log₁₀(Ka), where Ka is the acid dissociation constant. The inverse formula is Ka = 10^(−pKa). For example, if Ka = 1.8 × 10⁻⁵ (as for acetic acid), then pKa = −log₁₀(1.8 × 10⁻⁵) = −(log 1.8 + log 10⁻⁵) = −(0.255 − 5) = 4.745 ≈ 4.74. The negative logarithm transforms very small Ka values into manageable positive numbers on the pKa scale.
pKb is the negative logarithm of the base dissociation constant Kb of the conjugate base. At 25°C, pKa + pKb = 14 (derived from Kw = Ka × Kb = 10⁻¹⁴). So if an acid has pKa = 4.74, its conjugate base has pKb = 9.26. This relationship links every acid-conjugate base pair — knowing pKa immediately gives you pKb and Kb for the conjugate base.
pKa is a property of the acid itself — it does not change with concentration. pH is a property of the solution — it changes with concentration, temperature, and dilution. The Henderson-Hasselbalch equation links the two: pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA] (equal concentrations of acid and conjugate base), pH = pKa. This is why pKa is used to choose the right acid for preparing a buffer at a target pH.
Strong acids (HCl, H₂SO₄, HNO₃) have pKa values below 0 — they dissociate completely and Ka is very large, making pKa negative. Weak acids (acetic acid pKa 4.74, carbonic acid pKa₁ 6.35, ammonium ion pKa 9.25) have pKa values between 0 and 14. The lower the pKa, the stronger the weak acid. An acid is generally considered 'strong' if pKa < 0, and 'very weak' if pKa > 10.
Enter the Ka value of the weak acid into the Acid Dissociation Constant (Ka) field in decimal or scientific notation. The calculator returns pKa, pKb, Kb of the conjugate base, and an acid strength classification. The steps panel shows the logarithm working — useful for including in a JEE or NEET answer. For the reverse — finding Ka from pKa — use Ka = 10^(−pKa).
When preparing a buffer, the ideal weak acid has a pKa within one pH unit of the target buffer pH. At pKa = target pH, equal concentrations of acid and conjugate base give maximum buffer capacity (ratio [A⁻]/[HA] = 1). For example, to prepare a pH 4.7 acetate buffer, acetic acid (pKa 4.74) is an excellent choice because the target pH is within the effective buffer range of pKa ± 1 = 3.74–5.74. Use the Buffer pH Calculator alongside this tool.
Common acids and their pKa values: acetic acid (CH₃COOH) = 4.76, oxalic acid (first) = 1.25, oxalic acid (second) = 4.27, citric acid (first) = 3.13, phosphoric acid (H₃PO₄, first) = 2.15, carbonic acid (H₂CO₃, first) = 6.35, ammonium ion (NH₄⁺) = 9.25, boric acid = 9.24, hydrogen carbonate (HCO₃⁻, second Ka) = 10.33. These values are essential for Class 11 equilibrium problems and for choosing buffers in undergraduate analytical chemistry labs.
Yes — strong acids have negative pKa values because their Ka is greater than 1. For example, HCl has Ka ≈ 10⁷, giving pKa ≈ −7. Perchloric acid (HClO₄) has pKa ≈ −10. In practice, pKa is only a useful concept for weak acids; for strong acids, dissociation is taken as complete and Ka is not measured. The pKa Calculator returns negative values for Ka > 1 inputs, but the acid strength label flags these as 'strong acid' cases.
pKa is introduced in NCERT Class 11 Chemistry Chapter 7 (Equilibrium) under the topic of weak acid equilibria and the dissociation constant Ka. Students are expected to convert Ka ↔ pKa and to use pKa in the Henderson-Hasselbalch equation for buffer problems. JEE Advanced regularly includes multi-step problems where Ka is given and students must find [H⁺], pH, degree of dissociation, and buffer pH in sequence. NEET tests pKa in the context of amino acid ionisation and biochemical pH regulation.
The degree of dissociation (α) of a weak acid at molar concentration C is approximately α = √(Ka/C) = √(10^(−pKa)/C) for small α. As pKa increases (weaker acid), α decreases for the same concentration. For example, acetic acid (pKa 4.74) at 0.1 M has α ≈ √(1.8 × 10⁻⁵ / 0.1) ≈ 0.013 = 1.3%. Knowing pKa and concentration, you can calculate the actual [H⁺] without assuming complete dissociation.
Also known as
Ka to pKaacid dissociation constant calculatorpKa pKb calculatorpKa from Kaweak acid Ka