How to Calculate Ka from pH

Buffer Approximation Equation:

\[ K_a = 10^{-pH} \times \frac{[HA]}{[A^-]} \]

Ka:

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1. What is the Buffer Approximation Equation?

The buffer approximation equation calculates the acid dissociation constant (Ka) from pH measurements and concentrations of acid and conjugate base. This method is particularly useful for weak acid buffer systems where the approximation holds true.

2. How Does the Calculator Work?

The calculator uses the buffer approximation equation:

\[ K_a = 10^{-pH} \times \frac{[HA]}{[A^-]} \]

Where:

\( K_a \) — Acid dissociation constant (M)
\( pH \) — Negative logarithm of hydrogen ion concentration (unitless)
\( [HA] \) — Concentration of weak acid (M)
\( [A^-] \) — Concentration of conjugate base (M)

Explanation: This equation derives from the Henderson-Hasselbalch equation rearranged to solve for Ka, assuming the activity coefficients are approximately 1.

3. Importance of Ka Calculation

Details: Ka values are fundamental in understanding acid strength, predicting buffer capacity, and designing buffer solutions for biochemical and analytical applications.

4. Using the Calculator

Tips: Enter pH value between 0-14, concentrations of acid [HA] and conjugate base [A⁻] in molarity (M). All concentration values must be positive.

5. Frequently Asked Questions (FAQ)

Q1: When is the buffer approximation valid?
A: The approximation works best when [HA] and [A⁻] are similar in magnitude and the acid is weak (Ka < 10⁻³).

Q2: What are typical Ka values for common acids?
A: Strong acids have Ka > 1, weak acids have Ka < 1. For example, acetic acid Ka ≈ 1.8×10⁻⁵, hydrochloric acid Ka ≈ 10⁷.

Q3: How does temperature affect Ka calculations?
A: Ka is temperature-dependent. Most tabulated values are at 25°C. Significant temperature variations may affect accuracy.

Q4: Are there limitations to this method?
A: This method assumes ideal behavior and may not be accurate for very concentrated solutions or when ionic strength effects are significant.

Q5: Can this be used for polyprotic acids?
A: For polyprotic acids, this method calculates the Ka for the specific protonation step being measured, but additional considerations are needed for complete characterization.