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Weak Acid pH Formula:
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The weak acid pH calculation estimates the pH of a weak acid solution using the acid dissociation constant (pKa) and molar concentration. This approximation is valid for weak acids where dissociation is minimal and simplifies the Henderson-Hasselbalch equation.
The calculator uses the weak acid approximation formula:
Where:
Explanation: This formula is derived from the equilibrium expression for weak acids and assumes that the concentration of H⁺ ions equals the square root of Ka × C.
Details: Accurate pH calculation is essential for understanding acid-base chemistry, predicting chemical behavior, and applications in biochemistry, pharmaceuticals, and environmental science.
Tips: Enter pKa value (typically between 0-14 for common weak acids) and acid concentration in molarity. Both values must be positive numbers.
Q1: When is this approximation valid?
A: This approximation works best for weak acids with pKa values typically above 3 and concentrations where the approximation [H⁺] ≈ √(Ka × C) holds true.
Q2: What are typical pKa values for common weak acids?
A: Acetic acid (4.76), formic acid (3.75), benzoic acid (4.20), hydrofluoric acid (3.17), and carbonic acid (6.35).
Q3: When should I use the exact Henderson-Hasselbalch equation?
A: Use the full Henderson-Hasselbalch equation when dealing with buffer solutions or when the weak acid concentration is very dilute.
Q4: What are the limitations of this calculation?
A: This approximation may not be accurate for very concentrated solutions, very dilute solutions, or when autoionization of water becomes significant.
Q5: How does temperature affect pH calculation?
A: Temperature affects both Ka values and the ionic product of water (Kw), so pKa values should be used at the appropriate temperature.