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What is Ka?
The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It is defined as the equilibrium constant for the dissociation of an acid (HA) into its conjugate base (A-) and a proton (H+):
$$ HA \rightleftharpoons H^+ + A^- $$
The expression for the equilibrium constant (Ka) is given by:
$$ Ka = \frac{[H^+][A^-]}{[HA]} $$
Where:
- [H+] is the concentration of hydrogen ions.
- [A-] is the concentration of the conjugate base.
- [HA] is the concentration of the undissociated acid.
A larger value of Ka indicates a stronger acid, as it implies more dissociation into H+ and A-.
What is pKa?
The pKa value is the negative base-10 logarithm of the Ka value:
$$ pKa = -\log_{10}(Ka) $$
The pKa provides a more convenient way to express acidity, as it typically results in more manageable numbers. A lower pKa value indicates a stronger acid, which corresponds to a higher Ka.
Relationship between Ka and pKa
The relationship between Ka and pKa can be described by the formula:
$$ Ka = 10^{-pKa} $$
This shows that Ka and pKa are inversely related: as Ka increases (indicating a stronger acid), pKa decreases.
How the Calculator Works
The Ka ↔️ pKa Calculator allows you to input either a Ka or a pKa value, and it will convert it to the corresponding pKa or Ka using the formulas above. Simply enter the known value and select the calculation direction to get the result.
Example Calculation
To convert a pKa value of 4.75 to Ka:
Use the formula: $$ Ka = 10^{-pKa} $$
Substitute pKa = 4.75: $$ Ka = 10^{-4.75} $$
Result: 1.78e-5
Related Concepts
pH and pKa
The pH of a solution is another measure that indicates its acidity or alkalinity. The pH is defined as:
$$ pH = -\log_{10}([H^+]) $$
There is a relationship between pH, pKa, and the concentration of the acid and its conjugate base in a solution. The Henderson-Hasselbalch equation provides this relationship:
$$ pH = pKa + \log_{10}\left(\frac{[A^-]}{[HA]}\right) $$
Buffer Solutions
Buffer solutions resist changes in pH upon the addition of small amounts of acid or base. They are made using a weak acid and its conjugate base (or a weak base and its conjugate acid).
