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P Ka Calculator

Calculate ka with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.

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Chemistry

P Ka Calculator

Calculate pKa from Ka, Ka from pKa, convert between pKa and pKb, and estimate pH of acid/base solutions. Supports acid dissociation constant conversions and strength classification.

Last updated: December 2025

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Formula

pKa = -log10(Ka) | pKa + pKb = 14 | pH = -log10(sqrt(Ka x C))

pKa is the negative logarithm of the acid dissociation constant Ka. For conjugate acid-base pairs at 25C, pKa + pKb = 14. The pH of a weak acid solution can be approximated using pH = -log10(sqrt(Ka x C)) when the acid is not too strong and concentration is not too dilute.

Last reviewed: December 2025

Worked Examples

Example 1: pKa of Acetic Acid

Acetic acid has Ka = 1.8 x 10^-5. Find its pKa and the pH of a 0.1 M solution.
Solution:
pKa = -log10(1.8 x 10^-5) = 4.74 pKb = 14 - 4.74 = 9.26 pH = -log10(sqrt(1.8e-5 x 0.1)) pH = -log10(1.342e-3) = 2.87
Result: pKa = 4.74, pH = 2.87

Example 2: Converting pKa to Ka

A weak acid has pKa = 3.17. Find Ka and classify its strength.
Solution:
Ka = 10^(-3.17) = 6.76 x 10^-4 pKb = 14 - 3.17 = 10.83 Kb = 10^(-10.83) = 1.48 x 10^-11 pKa between 0 and 4: Moderate acid
Result: Ka = 6.76 x 10^-4, Moderate Acid
Expert Insights

Background & Theory

The P Ka Calculator applies the following established principles and formulas. Chemistry is the science of matter's composition, structure, properties, and transformations. At the heart of quantitative chemistry lies the mole concept. One mole of any substance contains exactly 6.022×10²³ entities (Avogadro's number, Nₐ), and the molar mass of an element or compound in grams per mole is numerically equal to its atomic or molecular mass in atomic mass units. This allows chemists to convert between measurable mass and the number of reacting particles. Stoichiometry uses balanced chemical equations to relate the amounts of reactants and products. A balanced equation conserves both mass and charge. Molarity, the most common concentration unit, is defined as M = n/V, where n is moles of solute and V is volume of solution in liters, giving units of mol/L. Acidity and basicity are quantified by the pH scale, defined as pH = −log₁₀[H⁺], where [H⁺] is the molar concentration of hydrogen ions. Pure water at 25°C has pH 7.00; acids have lower values and bases higher values. Each unit change represents a tenfold change in hydrogen ion concentration. Gas behavior is described by the ideal gas law PV = nRT, where P is pressure in pascals, V is volume in cubic meters, n is moles, R = 8.314 J/(mol·K), and T is temperature in kelvin. Special cases include Boyle's Law (P₁V₁ = P₂V₂ at constant temperature) and Charles's Law (V₁/T₁ = V₂/T₂ at constant pressure). Thermochemistry quantifies heat changes in reactions through enthalpy, H. Hess's Law states that the total enthalpy change for a reaction is the sum of enthalpy changes for any sequence of steps leading to the same overall reaction, making it possible to calculate enthalpies for reactions that cannot be measured directly. Electron configuration describes the distribution of electrons in atomic orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. Periodic trends including atomic radius, ionization energy, and electronegativity arise systematically from electron configuration and nuclear charge, enabling chemists to predict and rationalize chemical behavior across the periodic table.

History

The history behind the P Ka Calculator traces back through the following developments. Chemistry's roots lie in alchemy, the medieval practice combining proto-scientific experimentation with mystical aims. Alchemists developed practical techniques including distillation, calcination, and the preparation of acids, building a body of empirical knowledge despite their theoretical misunderstandings. Modern chemistry is conventionally dated to Antoine Lavoisier (1743–1794), often called the father of modern chemistry. Lavoisier demonstrated the law of conservation of mass in 1789, showing that matter is neither created nor destroyed in chemical reactions. He identified oxygen's role in combustion, dismantling the phlogiston theory, and co-authored the first systematic chemical nomenclature, establishing the language still used today. John Dalton proposed the first modern atomic theory in 1803, asserting that all matter is composed of indivisible atoms, that atoms of the same element are identical in mass, and that compounds form from fixed ratios of different atoms. This provided a physical basis for Lavoisier's conservation law and Proust's law of definite proportions. Dmitri Mendeleev published his periodic table in 1869, arranging the 63 known elements by atomic mass and revealing repeating patterns of chemical behavior. He boldly left gaps for undiscovered elements and predicted their properties with remarkable accuracy, predictions confirmed by the subsequent discovery of gallium, scandium, and germanium. Ernest Rutherford's gold foil experiment in 1911 revealed the nuclear model of the atom: a tiny, dense, positively charged nucleus surrounded by electrons. Niels Bohr refined this in 1913 with a quantized model of electron orbits that explained the hydrogen emission spectrum. Quantum chemistry and molecular orbital theory, developed through the 1920s and 1930s, provided the full quantum mechanical description of chemical bonding. The latter 20th century saw the rise of computational chemistry, enabling molecular simulation at unprecedented scale. The green chemistry movement, articulated in the 12 Principles of Green Chemistry in 1998, reoriented the field toward sustainability, waste reduction, and benign chemical design, reflecting chemistry's growing awareness of its environmental responsibilities.

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Frequently Asked Questions

pKa is the negative base-10 logarithm of the acid dissociation constant (Ka): pKa = -log10(Ka). It measures the strength of an acid in solution. A lower pKa indicates a stronger acid that dissociates more completely in water. For example, hydrochloric acid has a pKa of about -7 (very strong), acetic acid has a pKa of 4.76 (weak acid), and water has a pKa of 15.7 (very weak acid). The pKa scale is logarithmic, so each unit change represents a tenfold change in acid strength.
For a conjugate acid-base pair in water at 25 degrees Celsius, pKa + pKb = 14 (since Ka x Kb = Kw = 1.0 x 10^-14). This means a strong acid (low pKa) has a weak conjugate base (high pKb), and vice versa. For example, acetic acid has pKa = 4.76, so its conjugate base (acetate ion) has pKb = 14 - 4.76 = 9.24, confirming that acetate is a weak base. This relationship is fundamental to understanding acid-base equilibria and buffer chemistry.
For a weak acid with concentration C and dissociation constant Ka, the approximate pH formula is: pH = -log10(sqrt(Ka x C)). This assumes the degree of dissociation is small compared to the initial concentration (less than about 5%). For example, 0.1 M acetic acid (Ka = 1.8 x 10^-5): pH = -log10(sqrt(1.8 x 10^-5 x 0.1)) = -log10(1.34 x 10^-3) = 2.87. If the approximation is not valid (high Ka or low concentration), you need to solve the full quadratic equation.
The Henderson-Hasselbalch equation is pH = pKa + log10([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] is the acid concentration. It is used extensively in buffer calculations and titration problems. At the half-equivalence point of a titration, [A-] = [HA], so pH = pKa. This equation works best when the ratio of conjugate base to acid is between 0.1 and 10 (within 1 pH unit of the pKa). It is essential for designing buffers in biological and chemical systems.
In biochemistry, pKa values determine the protonation state of amino acids, which affects protein structure and enzyme activity. At physiological pH (7.4), groups with pKa below 7.4 are mostly deprotonated, while those above are mostly protonated. In pharmacology, pKa governs drug absorption since only the uncharged form of a drug can cross cell membranes easily. A drug with a pKa near physiological pH will have significant amounts of both charged and uncharged forms, affecting its distribution throughout the body.
For multiplication and division, the result has the same number of significant figures as the measurement with the fewest. For addition and subtraction, round to the least number of decimal places. Exact numbers (counting, defined conversions) have infinite significant figures.
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Formula

pKa = -log10(Ka) | pKa + pKb = 14 | pH = -log10(sqrt(Ka x C))

pKa is the negative logarithm of the acid dissociation constant Ka. For conjugate acid-base pairs at 25C, pKa + pKb = 14. The pH of a weak acid solution can be approximated using pH = -log10(sqrt(Ka x C)) when the acid is not too strong and concentration is not too dilute.

Frequently Asked Questions

What is pKa and what does it tell you?

pKa is the negative base-10 logarithm of the acid dissociation constant (Ka): pKa = -log10(Ka). It measures the strength of an acid in solution. A lower pKa indicates a stronger acid that dissociates more completely in water. For example, hydrochloric acid has a pKa of about -7 (very strong), acetic acid has a pKa of 4.76 (weak acid), and water has a pKa of 15.7 (very weak acid). The pKa scale is logarithmic, so each unit change represents a tenfold change in acid strength.

How are pKa and pKb related?

For a conjugate acid-base pair in water at 25 degrees Celsius, pKa + pKb = 14 (since Ka x Kb = Kw = 1.0 x 10^-14). This means a strong acid (low pKa) has a weak conjugate base (high pKb), and vice versa. For example, acetic acid has pKa = 4.76, so its conjugate base (acetate ion) has pKb = 14 - 4.76 = 9.24, confirming that acetate is a weak base. This relationship is fundamental to understanding acid-base equilibria and buffer chemistry.

How do you calculate pH from Ka and concentration?

For a weak acid with concentration C and dissociation constant Ka, the approximate pH formula is: pH = -log10(sqrt(Ka x C)). This assumes the degree of dissociation is small compared to the initial concentration (less than about 5%). For example, 0.1 M acetic acid (Ka = 1.8 x 10^-5): pH = -log10(sqrt(1.8 x 10^-5 x 0.1)) = -log10(1.34 x 10^-3) = 2.87. If the approximation is not valid (high Ka or low concentration), you need to solve the full quadratic equation.

What is the Henderson-Hasselbalch equation and when is it used?

The Henderson-Hasselbalch equation is pH = pKa + log10([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] is the acid concentration. It is used extensively in buffer calculations and titration problems. At the half-equivalence point of a titration, [A-] = [HA], so pH = pKa. This equation works best when the ratio of conjugate base to acid is between 0.1 and 10 (within 1 pH unit of the pKa). It is essential for designing buffers in biological and chemical systems.

Why is pKa important in biochemistry and pharmacology?

In biochemistry, pKa values determine the protonation state of amino acids, which affects protein structure and enzyme activity. At physiological pH (7.4), groups with pKa below 7.4 are mostly deprotonated, while those above are mostly protonated. In pharmacology, pKa governs drug absorption since only the uncharged form of a drug can cross cell membranes easily. A drug with a pKa near physiological pH will have significant amounts of both charged and uncharged forms, affecting its distribution throughout the body.

How do significant figures apply to chemistry calculations?

For multiplication and division, the result has the same number of significant figures as the measurement with the fewest. For addition and subtraction, round to the least number of decimal places. Exact numbers (counting, defined conversions) have infinite significant figures.

References

Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy