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Weak Acid Concentration Formula:
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The calculation of weak acid concentration from pH and acid dissociation constant (K_a) is a fundamental concept in acid-base chemistry. It allows determination of the initial concentration of a weak acid based on its measured pH and known dissociation constant.
The calculator uses the weak acid concentration formula:
Where:
Explanation: This formula approximates the concentration for weak acids where the degree of dissociation is small. The hydrogen ion concentration is calculated from pH, then divided by the acid dissociation constant to find the original acid concentration.
Details: Knowing the concentration of weak acids is crucial for buffer preparation, pharmaceutical formulations, environmental monitoring, and understanding chemical equilibrium in various industrial and laboratory applications.
Tips: Enter pH value between 0-14 and K_a value greater than 0. For accurate results, ensure pH measurement is precise and K_a value is known for the specific temperature conditions.
Q1: When is this approximation valid?
A: This approximation works best for weak acids where the degree of dissociation is less than 5% and when [H⁺] ≈ √(K_a × [HA]) assumption holds.
Q2: What are typical K_a values for common weak acids?
A: Acetic acid: 1.8×10⁻⁵, Formic acid: 1.8×10⁻⁴, Carbonic acid: 4.3×10⁻⁷, Hydrocyanic acid: 4.9×10⁻¹⁰.
Q3: How does temperature affect the calculation?
A: K_a values are temperature-dependent. Use K_a values measured at the same temperature as the pH measurement for accurate results.
Q4: Can this be used for polyprotic acids?
A: For polyprotic acids, the calculation becomes more complex and requires consideration of multiple dissociation steps and their respective K_a values.
Q5: What are the limitations of this method?
A: This method assumes ideal behavior, neglects activity coefficients, and may not be accurate for very dilute solutions or when ionic strength is high.