How To Calculate Pka From Ph And Concentration

Henderson-Hasselbalch Equation:

\[ pKa = pH - \log\left(\frac{[A^-]}{[HA]}\right) \]

Calculated pKa:

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1. What is the Henderson-Hasselbalch Equation?

The Henderson-Hasselbalch equation is a chemical equation that establishes the relationship between pH and pKa (acid dissociation constant) in buffer solutions. It provides a way to calculate the pH of a buffer solution or determine pKa from pH and concentration measurements.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

\[ pKa = pH - \log\left(\frac{[A^-]}{[HA]}\right) \]

Where:

\( pKa \) — Acid dissociation constant (dimensionless)
\( pH \) — Measure of acidity/alkalinity (dimensionless)
\( [A^-] \) — Concentration of conjugate base (mol/L)
\( [HA] \) — Concentration of weak acid (mol/L)

Explanation: The equation relates the pH of a solution to the pKa of the acid and the ratio of the concentrations of the conjugate base and the acid.

3. Importance of pKa Calculation

Details: Accurate pKa determination is crucial for understanding acid-base chemistry, predicting buffer capacity, pharmaceutical development, and biological systems where pH balance is critical.

4. Using the Calculator

Tips: Enter pH value (0-14), concentrations of conjugate base [A⁻] and weak acid [HA] in mol/L. All concentration values must be positive and non-zero.

5. Frequently Asked Questions (FAQ)

Q1: What is the significance of pKa in chemistry?
A: pKa indicates the strength of an acid - lower pKa values correspond to stronger acids. It helps predict acid-base behavior and buffer ranges.

Q2: When is the Henderson-Hasselbalch equation valid?
A: The equation is most accurate when the concentrations of [A⁻] and [HA] are similar and when dealing with weak acids where the approximation holds.

Q3: What are typical pKa values for common acids?
A: Acetic acid: ~4.76, Carbonic acid: ~6.3, Phosphoric acid: ~2.1, 7.2, 12.7 (for its three protons).

Q4: Can this equation be used for strong acids?
A: No, the Henderson-Hasselbalch equation is specifically designed for weak acids and their conjugate bases.

Q5: How does temperature affect pKa calculations?
A: pKa values are temperature-dependent. The equation assumes constant temperature, so measurements should be made at consistent temperatures for accurate results.