1. What is the Weak Acid pH Formula?

The weak acid pH formula calculates the pH of a weak acid solution using its acid dissociation constant (Ka) and concentration. The formula is derived from the equilibrium expression for weak acid dissociation.

2. How Does the Calculator Work?

The calculator uses the weak acid pH formula:

Where:

• \( pKa \) — Negative logarithm of the acid dissociation constant (dimensionless)
• \( C \) — Concentration of the weak acid (mol/L)

Explanation: This formula provides an approximation for the pH of a weak acid solution, assuming the concentration of H⁺ ions from water dissociation is negligible compared to that from the acid.

3. Importance of pH Calculation

Details: Accurate pH calculation is crucial for understanding acid-base chemistry, predicting chemical behavior, and applications in biochemistry, environmental science, and industrial processes.

4. Using the Calculator

Tips: Enter pKa value (dimensionless) and concentration in mol/L. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the range of validity for this formula?
A: This formula works best for weak acids where the approximation [H⁺] ≈ √(Ka·C) is valid, typically when Ka < 10⁻² and C > 10⁻⁶ M.

Q2: How is pKa related to Ka?
A: pKa = -log₁₀(Ka), where Ka is the acid dissociation constant. A lower pKa value indicates a stronger acid.

Q3: When is this approximation not accurate?
A: The formula may be less accurate for very dilute solutions or very weak acids where water's autoionization contributes significantly to the H⁺ concentration.

Q4: Can this formula be used for polyprotic acids?
A: This formula is specifically for monoprotic weak acids. Polyprotic acids require more complex calculations that account for multiple dissociation steps.

Q5: What are typical pKa values for common weak acids?
A: Acetic acid: pKa ≈ 4.76, Formic acid: pKa ≈ 3.75, Carbonic acid: pKa₁ ≈ 6.3, Phosphoric acid: pKa₁ ≈ 2.1.