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pH Formula for Weak Acids:
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The pH formula using Ka calculates the pH of a weak acid solution based on its acid dissociation constant (Ka) and concentration. This formula is essential for understanding acid-base chemistry and predicting the acidity of weak acid solutions.
The calculator uses the weak acid pH formula:
Where:
Explanation: For weak acids that only partially dissociate, the hydrogen ion concentration can be approximated using the square root of the product of Ka and the initial concentration.
Details: Accurate pH calculation is crucial for chemical reactions, biological systems, industrial processes, and environmental monitoring. Understanding weak acid behavior helps in buffer preparation and acid-base titration.
Tips: Enter the acid dissociation constant (Ka) and the concentration of the weak acid in moles per liter (M). Both values must be positive numbers greater than zero.
Q1: What is the difference between strong and weak acids?
A: Strong acids completely dissociate in water, while weak acids only partially dissociate. This formula applies specifically to weak acids.
Q2: When is this approximation valid?
A: This approximation works well when the acid is weak (Ka < 10⁻³) and the concentration is not extremely dilute.
Q3: What are typical Ka values for common weak acids?
A: Acetic acid: 1.8×10⁻⁵, Formic acid: 1.8×10⁻⁴, Hydrofluoric acid: 6.8×10⁻⁴, Carbonic acid: 4.3×10⁻⁷.
Q4: Why use the square root in the calculation?
A: The square root comes from the equilibrium expression for weak acid dissociation: Ka = [H⁺]²/C for monoprotic acids when [H⁺] = [A⁻].
Q5: What if the acid concentration is very low?
A: For very dilute solutions, you may need to account for the autoionization of water, especially when [H⁺] from the acid approaches 10⁻⁷ M.