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pH Equation at Equivalence Point:
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The equation \( pH = \frac{1}{2} pK_w + \frac{1}{2} pK_a + \frac{1}{2} \log C \) calculates the pH at the equivalence point in a weak acid-strong base titration. This point occurs when the amount of base added equals the amount of acid initially present.
The calculator uses the equivalence point pH equation:
Where:
Explanation: At equivalence point, the solution contains the conjugate base of the weak acid, making the pH dependent on the hydrolysis of this salt.
Details: Accurate pH calculation at equivalence point is crucial for determining the appropriate indicator choice in acid-base titrations and understanding the titration curve behavior.
Tips: Enter pKw (typically 14.00), pKa of the weak acid, and concentration of the resulting salt. All values must be positive numbers.
Q1: Why is the equivalence point pH not 7 for weak acid-strong base titration?
A: Because the salt formed hydrolyzes to produce a basic solution, making the pH > 7 at equivalence point.
Q2: What is the typical range of pKa values for weak acids?
A: Weak acids typically have pKa values between 2-12, with most common weak acids in the 3-7 range.
Q3: How does temperature affect pKw?
A: pKw decreases with increasing temperature (e.g., 13.99 at 37°C, 13.40 at 60°C).
Q4: When is this equation not applicable?
A: This equation assumes ideal behavior, dilute solutions, and that the concentration C represents the salt concentration at equivalence point.
Q5: What indicators are suitable for weak acid-strong base titrations?
A: Indicators with transition ranges in the basic pH region (e.g., phenolphthalein, pH 8.3-10.0) are typically used.