Henderson-Hasselbalch Equation Calculator

Henderson-Hasselbalch Equation:

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

Acid Dissociation Constant (pKa):

unitless

Conjugate Base Concentration ([A⁻]):

mol/L

Weak Acid Concentration ([HA]):

mol/L

Calculated pH:

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1. What is the Henderson-Hasselbalch Equation?

The Henderson-Hasselbalch equation is used to calculate the pH of a solution containing a weak acid and its conjugate base. It provides a convenient way to estimate pH in buffer solutions and is fundamental in acid-base chemistry.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

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

Where:

\( pH \) — Negative logarithm of hydrogen ion concentration (unitless)
\( pKa \) — Negative logarithm of acid dissociation constant (unitless)
\( [A^-] \) — Concentration of conjugate base (mol/L)
\( [HA] \) — Concentration of weak acid (mol/L)

Explanation: The equation relates pH, pKa, and the ratio of conjugate base to weak acid concentrations, providing insight into buffer capacity and acid-base equilibrium.

3. Importance of pH Calculation

Details: Accurate pH calculation is crucial for understanding chemical reactions, biological systems, pharmaceutical formulations, and environmental chemistry. It helps predict the behavior of weak acids and bases in solution.

4. Using the Calculator

Tips: Enter pKa value, conjugate base concentration in mol/L, and weak acid concentration in mol/L. All concentrations must be positive values, and weak acid concentration must be greater than zero.

5. Frequently Asked Questions (FAQ)

Q1: What is the range of validity for the Henderson-Hasselbalch equation?
A: The equation is most accurate when the concentrations of weak acid and conjugate base are similar and when the pH is within ±1 unit of the pKa.

Q2: Can this equation be used for strong acids?
A: No, the Henderson-Hasselbalch equation is specifically designed for weak acids and their conjugate bases. Strong acids are completely dissociated in solution.

Q3: What are typical pKa values for common weak acids?
A: Acetic acid (4.76), phosphoric acid (2.15, 7.20, 12.32), carbonic acid (6.35, 10.33). Values vary depending on the specific acid.

Q4: How does temperature affect the calculation?
A: Temperature affects both pKa values and the autoionization constant of water (Kw). For precise work, pKa values should be adjusted for temperature.

Q5: What is the significance of the ratio [A⁻]/[HA]?
A: When [A⁻] = [HA], pH = pKa. The ratio determines the buffer capacity and the position of the acid-base equilibrium.