P Ka Calculator
Calculate ka with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Manoj Kumar, Mathematics Educator
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.
Worked Examples
Example 1: pKa of Acetic Acid
Problem: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\npKb = 14 - 4.74 = 9.26\npH = -log10(sqrt(1.8e-5 x 0.1))\npH = -log10(1.342e-3) = 2.87
Result:pKa = 4.74, pH = 2.87
Example 2: Converting pKa to Ka
Problem:A weak acid has pKa = 3.17. Find Ka and classify its strength.
Solution:Ka = 10^(-3.17) = 6.76 x 10^-4\npKb = 14 - 3.17 = 10.83\nKb = 10^(-10.83) = 1.48 x 10^-11\npKa between 0 and 4: Moderate acid
Result:Ka = 6.76 x 10^-4, Moderate Acid
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.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy