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Weak Acid Dissociation:
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Weak acid pH calculation involves determining the acidity of a solution containing a weak acid using its dissociation constant (Ka) and concentration. Weak acids only partially dissociate in water, unlike strong acids that dissociate completely.
The calculator uses the weak acid dissociation equations:
Where:
Explanation: For weak acids where Ka is small, we can approximate that [H+] = [A-] and [HA] remains nearly constant, leading to the simplified calculation.
Details: Accurate pH calculation is essential in chemistry, biology, medicine, and environmental science. It helps predict chemical behavior, biological activity, and environmental impact of acidic solutions.
Tips: Enter the acid dissociation constant (Ka) and the initial concentration of the weak acid in mol/L. Both values must be positive numbers. The calculator assumes the weak acid approximation is valid.
Q1: What is the weak acid approximation?
A: The assumption that for weak acids with small Ka values, the dissociation is minimal, so [HA] ≈ initial concentration and [H+] = [A-] = sqrt(Ka × [HA]).
Q2: When is the weak acid approximation valid?
A: When Ka < 0.01 and concentration is not extremely dilute. For very dilute solutions or stronger acids, more complex calculations are needed.
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: How does temperature affect Ka and pH?
A: Ka values are temperature-dependent. Most weak acids have Ka values that increase with temperature, affecting the calculated pH.
Q5: What if my acid is polyprotic?
A: For polyprotic acids (like H₂SO₄, H₃PO₄), only the first dissociation is typically considered for weak acid calculations, unless subsequent Ka values are significant.