pKa Calculator
ChemistryCalculate pKa from acid dissociation constant Ka, or from pH and the concentrations of acid and conjugate base. Includes pKb and Kb outputs.
Reviewed by the thecalcu.com team · Last updated July 3, 2026
pKa
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.
Why Use a pKa Calculator?
Converting Ka to pKa requires taking a base-10 logarithm of a number expressed in scientific notation, for example, log₁₀(1.8 × 10⁻⁵). This requires splitting the logarithm: log(1.8) + log(10⁻⁵) = 0.2553 + (−5) = −4.7447, then negating to get pKa = 4.7447. Under exam conditions or when working with multiple acids, this is error-prone.
Key use cases:
- Buffer selection: Instantly assess whether an acid's pKa is suitable for a target buffer pH without manual log calculation.
- Conjugate base strength: Use pKb output to assess how much the conjugate base will hydrolyse in water, important for predicting the pH of salt solutions.
- JEE/NEET problem verification: Confirm pKa as an intermediate step before applying Henderson-Hasselbalch or titration equations.
- Acid strength comparison: Compare pKa values across a series of acids to understand substituent effects in organic chemistry.
The pH Calculator is the companion tool, once pKa is known and [H⁺] is calculated from Ka and concentration, use it to convert [H⁺] to pH.
Who Should Use This Calculator?
Class 11 chemistry students working through the equilibrium chapter, where Ka-to-pKa conversions are a basic skill. The pKa Calculator also returns pKb and Kb automatically, saving the separate calculation of the conjugate base equilibrium constant.
JEE Main and Advanced aspirants who need pKa as an input to Henderson-Hasselbalch buffer problems or to compare acid strengths in a series of compounds. JEE Advanced often provides Ka values and asks for pKa, pH, or buffer ratio in a chain.
NEET students who encounter pKa in the context of amino acid side chains (each amino acid has two or three pKa values for its ionisable groups) and protein isoelectric points. Understanding pKa is prerequisite to using the Henderson-Hasselbalch Calculator for biological buffer problems.
Undergraduate chemistry and biochemistry students preparing buffers in the lab. The pKa output tells you which acid to use; the Buffer pH Calculator tells you the ratio to mix.
Pharmaceutical chemistry and formulation students where drug pKa values determine absorption (since only the un-ionised form crosses lipid membranes), solubility, and stability, understanding how to move between Ka and pKa is essential.
What Insights Does the pKa Calculator Give You?
pKa is the primary highlighted output, the negative log of Ka. A pKa of 4.74 means the acid is moderately weak, half-dissociated at pH 4.74. Use this to immediately place the acid on the strength spectrum: pKa 0–3 = moderately strong weak acid, 3–7 = weak acid, 7–11 = very weak acid, >11 = extremely weak acid.
pKb shows the basicity of the conjugate base. Calculated as 14 − pKa, this tells you how strongly the conjugate base reacts with water. A pKb of 9.26 (for acetate, from acetic acid pKa 4.74) means acetate is a very weak base, it hydrolyses water only slightly, explaining why sodium acetate solutions are mildly basic (pH ~8.9 for 0.1 M).
Kb is the base dissociation constant of the conjugate base in mol/L, the numeric value corresponding to pKb. This is what goes into Kb equilibrium expressions for calculating [OH⁻] produced by hydrolysis: [OH⁻] = √(Kb × C) for dilute solutions.
Acid Strength classification provides a qualitative label based on the pKa range, contextualising the number for quick interpretation in lab reports or problem write-ups.
How to use this pKa calculator
- 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.
- 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.
- 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.
- Read pKb, the conjugate base's pKb = 14 − pKa. Use this to assess how basic the conjugate salt will be in water.
- Read Kb, the numeric base dissociation constant for the conjugate base. Use this in Kb equilibrium expressions.
- 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.
Show formula & methodology ↓Show less ↑
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.
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