Calculate Ka From pH Weak Acid

Ka Calculation Formula:

\[ K_a = 10^{-pK_a} \] \[ pK_a = pH + \log\left(\frac{[HA]}{[A^-]}\right) \]

Acid Dissociation Constant (Ka):

pKa Value:

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1. What is Ka for Weak Acid?

The acid dissociation constant (Ka) quantifies the strength of a weak acid in solution. It represents the equilibrium constant for the dissociation reaction: HA ⇌ H⁺ + A⁻. Smaller Ka values indicate weaker acids, while larger values indicate stronger acids.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

\[ K_a = 10^{-pK_a} \] \[ pK_a = pH + \log\left(\frac{[HA]}{[A^-]}\right) \]

Where:

\( K_a \) — Acid dissociation constant (unitless)
\( pK_a \) — Negative logarithm of Ka
\( pH \) — Measured pH of the solution
\( [HA] \) — Concentration of weak acid (M)
\( [A^-] \) — Concentration of conjugate base (M)

Explanation: The equation relates pH to the ratio of conjugate acid-base pair concentrations, allowing calculation of the acid dissociation constant.

3. Importance of Ka Calculation

Details: Ka values are essential for understanding acid strength, predicting buffer capacity, calculating pH of solutions, and designing chemical processes involving weak acids.

4. Using the Calculator

Tips: Enter pH value (0-14), concentrations of weak acid [HA] and conjugate base [A⁻] in molarity (M). Ensure all values are positive and concentrations are in correct units.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between Ka and pKa?
A: Ka is the acid dissociation constant, while pKa is its negative logarithm (pKa = -log₁₀Ka). pKa provides a more convenient scale for comparing acid strengths.

Q2: What are typical Ka values for weak acids?
A: Weak acids typically have Ka values between 10⁻² and 10⁻¹⁰. For example, acetic acid has Ka ≈ 1.8×10⁻⁵ (pKa = 4.76).

Q3: When is this calculation most accurate?
A: This method works best for monoprotic weak acids in dilute solutions where activity coefficients are close to 1 and temperature is around 25°C.

Q4: Can I use this for strong acids?
A: No, strong acids completely dissociate in water, so their Ka values are very large and this calculation method doesn't apply.

Q5: How does temperature affect Ka values?
A: Ka values are temperature-dependent. Most reported Ka values are for 25°C, and significant temperature variations can affect the results.