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Ka Formula:
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The Ka from pH calculation approximates the acid dissociation constant (Kₐ) from the pH of a weak acid solution, assuming low dissociation. This method provides a quick estimate of acid strength based on measurable pH values.
The calculator uses the formula:
Where:
Explanation: This formula assumes the weak acid dissociates only slightly, making the approximation valid for most weak acids where [H⁺] is much smaller than C.
Details: Calculating Kₐ from pH is essential for determining acid strength, predicting acid-base behavior in solutions, and understanding chemical equilibrium in aqueous systems.
Tips: Enter pH value (0-14) and acid concentration in mol/L. Ensure concentration is greater than [H⁺] concentration for valid results. The calculator assumes standard temperature conditions.
Q1: When is this approximation valid?
A: This approximation works best for weak acids where dissociation is less than 5% and [H⁺] is significantly smaller than C.
Q2: What are typical Kₐ values for common acids?
A: Strong acids have Kₐ > 1, weak acids have Kₐ < 1. For example, acetic acid Kₐ ≈ 1.8×10⁻⁵, hydrochloric acid Kₐ ≈ 10⁷.
Q3: Why does the calculator show "Invalid: [H⁺] ≥ C"?
A: This occurs when the calculated [H⁺] equals or exceeds the total concentration, which violates the weak acid assumption and indicates invalid input data.
Q4: Can this be used for polyprotic acids?
A: No, this calculator is designed for monoprotic weak acids. Polyprotic acids require more complex calculations accounting for multiple dissociation steps.
Q5: How does temperature affect the results?
A: Kₐ values are temperature-dependent. This calculator assumes standard room temperature (25°C). For precise work, temperature corrections may be necessary.