How to Calculate pH of Weak Acid

pH Approximation Formula for Dilute Weak Acid:

\[ pH = \frac{1}{2} pK_a - \frac{1}{2} \log [HA] \]

pH:

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1. What is the pH Approximation for Weak Acid?

The pH approximation formula for dilute weak acid solutions provides a simplified method to estimate pH based on acid dissociation constant (pKa) and initial acid concentration ([HA]). This approximation is valid for dilute solutions where the degree of dissociation is small.

2. How Does the Calculator Work?

The calculator uses the pH approximation formula:

\[ pH = \frac{1}{2} pK_a - \frac{1}{2} \log [HA] \]

Where:

\( pH \) — Negative logarithm of hydrogen ion concentration (unitless)
\( pK_a \) — Negative logarithm of acid dissociation constant (unitless)
\( [HA] \) — Initial concentration of weak acid (M)

Explanation: This formula is derived from the equilibrium expression for weak acid dissociation and assumes that the concentration of dissociated ions is negligible compared to the initial acid concentration.

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 pKa value (unitless) and initial acid concentration in molarity (M). Both values must be positive numbers. The approximation works best for dilute solutions with concentrations typically below 0.1 M.

5. Frequently Asked Questions (FAQ)

Q1: When is this approximation valid?
A: This approximation is valid for dilute solutions of weak acids where the degree of dissociation is small (typically less than 5%).

Q2: What are typical pKa values for common weak acids?
A: Acetic acid: 4.76, Formic acid: 3.75, Carbonic acid: 6.35, Hydrofluoric acid: 3.17.

Q3: When should I use the exact calculation instead?
A: Use exact calculation methods for concentrated solutions, strong acids, or when higher precision is required.

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

Q5: What are the limitations of this approximation?
A: It doesn't account for activity coefficients, temperature effects, or the presence of other ions that might affect the equilibrium.