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pH Calculation for Weak Acid:
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The pH calculation from Ka (acid dissociation constant) and concentration allows estimation of pH for weak acid solutions. This method uses the approximation that for weak acids, the hydrogen ion concentration can be estimated from the square root of the product of Ka and the acid concentration.
The calculator uses the weak acid pH approximation formula:
Where:
Explanation: This approximation assumes that the acid is weak enough that the dissociation is small compared to the initial concentration, and that the autoionization of water can be neglected.
Details: Accurate pH calculation is essential in chemistry, biochemistry, environmental science, and industrial processes. It helps predict acid behavior, buffer capacity, and chemical reactivity in various solutions.
Tips: Enter Ka value in molar units (M), acid concentration in molar units (M). Both values must be positive numbers. The approximation works best for weak acids where Ka is significantly less than 1.
Q1: When is this approximation valid?
A: This approximation is valid for weak acids where Ka << C and when the calculated [H+] is much greater than 10^-7 M (contribution from water autoionization).
Q2: 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.
Q3: When should I use the exact quadratic formula instead?
A: Use the exact method when the acid is not very weak, when concentration is very low, or when higher precision is required for strong weak acids.
Q4: Does this work for polyprotic acids?
A: This simple approximation is primarily for monoprotic weak acids. Polyprotic acids require more complex calculations considering multiple dissociation steps.
Q5: What is the pH range where this approximation is most accurate?
A: The approximation works best in the pH range of approximately 3-6 for typical weak acid concentrations.