How To Calculate Ka Given Ph And Molarity

Acid Dissociation Constant Formula:

\[ K_a = \frac{[H^+] \times [A^-]}{[HA]} \approx \frac{(10^{-pH})^2}{C - 10^{-pH}} \]

Acid Dissociation Constant (Ka):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What Is The Acid Dissociation Constant?

The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution. It represents the equilibrium constant for the dissociation reaction of an acid: HA ⇌ H⁺ + A⁻. A larger Ka value indicates a stronger acid that dissociates more completely in water.

2. How Does The Calculator Work?

The calculator uses the acid dissociation constant formula:

\[ K_a = \frac{[H^+] \times [A^-]}{[HA]} \approx \frac{(10^{-pH})^2}{C - 10^{-pH}} \]

Where:

\( K_a \) — Acid dissociation constant (mol/L)
\( [H^+] \) — Hydrogen ion concentration = \( 10^{-pH} \)
\( [A^-] \) — Conjugate base concentration ≈ \( [H^+] \)
\( [HA] \) — Undissociated acid concentration ≈ \( C - [H^+] \)
\( C \) — Total acid concentration (mol/L)
\( pH \) — Negative logarithm of hydrogen ion concentration

Explanation: This approximation assumes that for weak acids, the hydrogen ion concentration is much smaller than the total acid concentration, and that [H⁺] ≈ [A⁻].

3. Importance Of Ka Calculation

Details: Calculating Ka is essential for understanding acid strength, predicting acid-base behavior in chemical reactions, determining buffer capacities, and designing chemical processes in industrial and laboratory settings.

4. Using The Calculator

Tips: Enter pH value (0-14) and total acid concentration in mol/L. Ensure the pH value is appropriate for a weak acid (typically between 2-6 for common weak acids) and that the concentration is positive.

5. Frequently Asked Questions (FAQ)

Q1: What is the range of Ka values for weak acids?
A: Weak acids typically have Ka values between 10⁻² and 10⁻¹⁰ mol/L. Strong acids have Ka values much greater than 1.

Q2: Why is this an approximation formula?
A: This formula assumes [H⁺] = [A⁻] and neglects water's autoionization. For very dilute solutions or very weak acids, more precise calculations may be needed.

Q3: How does temperature affect Ka values?
A: Ka values are temperature-dependent. Most dissociation constants are reported at 25°C, and values may change significantly with temperature variations.

Q4: Can this calculator be used for polyprotic acids?
A: No, this calculator is designed for monoprotic weak acids. Polyprotic acids have multiple Ka values for each dissociation step.

Q5: What is pKa and how is it related to Ka?
A: pKa = -log₁₀(Ka). It provides a more convenient scale where smaller pKa values indicate stronger acids.