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