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Weak Acid pH Formula:
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The weak acid pH formula \( pH = -\log \sqrt{K_a C} \) provides an approximation for calculating the pH of a weak acid solution using the acid dissociation constant (Ka) and concentration (C). This approximation is valid when the acid is sufficiently weak and the concentration is not too dilute.
The calculator uses the weak acid approximation formula:
Where:
Explanation: This formula is derived from the equilibrium expression for weak acid dissociation and makes the approximation that the concentration of H⁺ ions equals \(\sqrt{K_a C}\).
Details: Accurate pH calculation is essential for understanding acid-base chemistry, predicting chemical behavior, and applications in various fields including biochemistry, environmental science, and industrial processes.
Tips: Enter the acid dissociation constant (Ka) in mol/L and the concentration (C) in mol/L. Both values must be positive numbers greater than zero for valid calculation.
Q1: When is this approximation valid?
A: This approximation works best when Ka << C and the acid is sufficiently weak (typically Ka < 10⁻³).
Q2: What are the limitations of this formula?
A: The formula may not be accurate for very dilute solutions, strong acids, or when the approximation conditions are not met.
Q3: How does temperature affect the calculation?
A: Temperature affects the Ka value. The calculation should use Ka values measured at the appropriate temperature.
Q4: Can this formula be used for polyprotic acids?
A: For polyprotic acids, the calculation is more complex and typically only the first dissociation is considered for weak polyprotic acids.
Q5: What is the typical range of Ka values for weak acids?
A: Weak acids typically have Ka values between 10⁻² and 10⁻¹⁰, with most common weak acids falling in the 10⁻³ to 10⁻⁶ range.