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Weak Acid pH Formula:
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The weak acid pH approximation formula \( pH = \frac{1}{2} pKa - \frac{1}{2} \log C \) provides an estimate of pH for dilute weak acid solutions where the degree of dissociation (α) is much less than 1. This simplification is valid when the acid is sufficiently weak and dilute.
The calculator uses the weak acid approximation formula:
Where:
Explanation: This formula assumes that the weak acid is only slightly dissociated and that the contribution of H⁺ from water is negligible compared to that from the acid.
Details: Accurate pH calculation is essential for understanding acid-base chemistry, predicting chemical behavior, and applications in biochemistry, environmental science, and industrial processes.
Tips: Enter the pKa value (dimensionless) and concentration in mol/L. Ensure the concentration is positive and the solution conditions meet the approximation criteria (dilute weak acid with α << 1).
Q1: When is this approximation valid?
A: This approximation is valid for dilute weak acids where the degree of dissociation α is much less than 1, typically when C >> Ka.
Q2: What are the limitations of this formula?
A: This formula becomes inaccurate for stronger acids, concentrated solutions, or when the approximation α << 1 is not satisfied.
Q3: How does temperature affect the calculation?
A: Temperature affects both pKa values and the ionic product of water. The formula assumes standard temperature conditions (25°C).
Q4: Can this be used for polyprotic acids?
A: This formula is designed for monoprotic weak acids. Polyprotic acids require more complex calculations that account for multiple dissociation steps.
Q5: What if the calculated pH is close to 7?
A: If the calculated pH is near neutral (pH ≈ 7), the approximation may break down as the contribution from water autoionization becomes significant.