1. What is the Weak Acid pH Formula?

The weak acid pH formula calculates the pH of a weak acid solution from its acid dissociation constant (Ka) and concentration (C). This formula 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 formula:

Where:

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

Explanation: The formula 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 reaction outcomes, and applications in biochemistry, environmental science, and industrial processes.

4. Using the Calculator

Tips: Enter Ka in mol/L and concentration in mol/L. Both values must be positive numbers. The formula is valid for weak acids where Ka << C.

5. Frequently Asked Questions (FAQ)

Q1: When is this approximation valid?
A: This approximation works best for weak acids where Ka < 10⁻³ and the concentration is not extremely dilute.

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.

Q3: When should I use the exact quadratic formula instead?
A: Use the exact solution when Ka > 10⁻³ or when dealing with very dilute solutions where the approximation may not hold.

Q4: Does this work for polyprotic acids?
A: This formula is for monoprotic weak acids. Polyprotic acids require more complex calculations considering multiple dissociation steps.

Q5: How does temperature affect the calculation?
A: Ka values are temperature-dependent. The calculation assumes the Ka value provided is for the temperature of interest.