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Weak Acid Dissociation Formula:
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The acid dissociation constant (Ka) calculation from pH and molarity allows determination of weak acid strength using pH measurements and initial concentration. This method provides a practical approach to quantify acid strength in laboratory settings.
The calculator uses the weak acid approximation formula:
Where:
Explanation: The formula derives from the equilibrium expression for weak acid dissociation: HA ⇌ H⁺ + A⁻, using the approximation that [H⁺] = [A⁻] for monoprotic acids.
Details: Ka values are fundamental in acid-base chemistry, helping predict acid strength, buffer capacity, and reaction behavior. Accurate Ka determination is essential for pharmaceutical formulations, environmental chemistry, and biochemical applications.
Tips: Enter pH value (0-14) and initial acid concentration in molarity (M). Ensure pH is measured accurately and initial concentration is known precisely for reliable results.
Q1: When is the weak acid approximation valid?
A: The approximation works best when Ka < 10⁻³ and initial concentration is reasonably high (> 0.01 M), ensuring [H⁺] is much smaller than C_initial.
Q2: What are typical Ka values for common weak acids?
A: Acetic acid: ~1.8×10⁻⁵, Formic acid: ~1.8×10⁻⁴, Carbonic acid: ~4.3×10⁻⁷. Stronger weak acids have higher Ka values.
Q3: How does temperature affect Ka calculations?
A: Ka is temperature-dependent. Most tabulated values are at 25°C. Significant temperature variations may affect accuracy.
Q4: Can this method be used for polyprotic acids?
A: For polyprotic acids, the calculation becomes more complex as multiple equilibria must be considered separately.
Q5: What are common sources of error in Ka determination?
A: pH meter calibration, temperature fluctuations, ionic strength effects, and assumption validity are common error sources.