Science & Lab Tools
pKa Calculator
Calculate acid dissociation constant (pKa) and understand acid strength
Enter values to see results
Related to pKa Calculator
The pKa calculator helps determine the acid dissociation constant (Ka) and its negative logarithm (pKa) for acids in solution. These values are crucial in understanding acid strength and predicting acid-base reactions. The calculator uses three main formulas depending on the type of calculation needed:
pKa to Ka Conversion
Ka = 10^(-pKa)
This formula converts the pKa value to the acid dissociation constant (Ka). The Ka value represents the extent of dissociation of an acid in water.
Ka to pKa Conversion
pKa = -log10(Ka)
This formula converts the acid dissociation constant (Ka) to its negative logarithm (pKa). The pKa scale is more convenient for comparing acid strengths.
pH to pKa Calculation
pKa = pH + log10([HA]/[A-])
This formula, derived from the Henderson-Hasselbalch equation, calculates pKa using the pH and the ratio of acid concentration [HA] to its conjugate base [A-].
Understanding pKa and Ka values is essential for predicting acid behavior and strength. Here's how to interpret the results:
pKa Values and Acid Strength
• Lower pKa values (0-4) indicate strong acids that dissociate readily in water
• Medium pKa values (4-9) indicate moderate acids
• Higher pKa values (>9) indicate weak acids that dissociate less readily
The smaller the pKa value, the stronger the acid.
Ka Values
• Ka values are typically expressed in scientific notation (e.g., 1.0 × 10^-4)
• Larger Ka values indicate stronger acids
• Ka values less than 1 indicate weak acids
• Ka values greater than 1 indicate strong acids
1. What is the relationship between pKa and acid strength?
The pKa value is inversely related to acid strength. A lower pKa indicates a stronger acid because it means the acid more readily donates protons in solution. For example, an acid with pKa = 2 is stronger than an acid with pKa = 5.
2. Why do we use both Ka and pKa values?
We use both because they serve different purposes. Ka values directly represent the equilibrium constant of acid dissociation, while pKa values provide a more convenient logarithmic scale for comparing acid strengths. The pKa scale is particularly useful because acid strengths can vary by many orders of magnitude.
3. How does temperature affect pKa values?
Temperature can affect pKa values because it influences the equilibrium constant of acid dissociation. Generally, as temperature increases, pKa values slightly decrease, indicating that acids become marginally stronger at higher temperatures. However, this effect is usually small for most practical purposes.
4. Can pKa values be negative?
Yes, pKa values can be negative. Negative pKa values indicate very strong acids that completely dissociate in water. For example, hydrochloric acid (HCl) has a pKa of about -7, indicating it is a very strong acid. The more negative the pKa, the stronger the acid.
5. What is the scientific source for this calculator?
This calculator is based on fundamental principles of acid-base chemistry and equilibrium constants established in physical chemistry. The calculations follow the standard equations derived from the Law of Mass Action and the Henderson-Hasselbalch equation, which are documented in standard chemistry textbooks and peer-reviewed literature. The mathematical relationships between Ka and pKa were first established by Lawrence Joseph Henderson (1908) and Karl Albert Hasselbalch (1916), and have been validated through countless experimental studies in physical chemistry. The equations and methods used are consistent with IUPAC (International Union of Pure and Applied Chemistry) standards for acid-base calculations.