1. What is the Weak Acid pH Equation?

The weak acid pH equation calculates the pH of a weak acid solution from its acid dissociation constant (Ka) and molar concentration (C). This equation is derived from the approximation for weak acids where [H⁺] ≈ √(Ka × C).

2. How Does the Calculator Work?

The calculator uses the weak acid pH equation:

Where:

• \( K_a \) — Acid dissociation constant (mol/L)
• \( C \) — Molar concentration of the weak acid (mol/L)

Explanation: The equation assumes the weak acid is only partially dissociated and the concentration of H⁺ ions comes primarily from the acid's dissociation.

3. Importance of pH Calculation

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

4. Using the Calculator

Tips: Enter Ka value in mol/L and concentration in mol/L. Both values must be positive numbers greater than zero for valid calculation.

5. Frequently Asked Questions (FAQ)

Q1: When is this approximation valid?
A: This approximation works best when Ka is small (weak acid) and concentration is not extremely dilute (C >> Ka).

Q2: What are typical Ka values for weak acids?
A: Weak acids typically have Ka values between 10⁻² and 10⁻¹⁰ mol/L. Strong acids have much larger Ka values.

Q3: When should I use the exact quadratic equation instead?
A: Use the exact solution when the acid is moderately strong or very dilute, or when higher precision is required.

Q4: Does this work for polyprotic acids?
A: This equation is for monoprotic weak acids. Polyprotic acids require more complex calculations.

Q5: How does temperature affect the calculation?
A: Ka values are temperature-dependent. Use Ka values measured at the temperature of interest for accurate results.