Weak Acid Strong Base Ph Calculation

Henderson-Hasselbalch Equation:

\[ pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right) \]

Acid Dissociation Constant (pK_a):

unitless

Conjugate Base Concentration ([A⁻]):

mol/L

Weak Acid Concentration ([HA]):

mol/L

Calculated pH:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Henderson-Hasselbalch Equation?

The Henderson-Hasselbalch equation is used to calculate the pH of a solution containing a weak acid and its conjugate base. It is particularly useful in titration curves, especially at the half-equivalence point where [A⁻] = [HA].

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

\[ pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right) \]

Where:

\( pH \) — pH of the solution (unitless)
\( pK_a \) — Acid dissociation constant (unitless)
\( [A^-] \) — Concentration of conjugate base (mol/L)
\( [HA] \) — Concentration of weak acid (mol/L)

Explanation: The equation relates the pH of a solution to the pK_a and the ratio of the concentrations of the conjugate base to the weak acid.

3. Importance of pH Calculation in Titration

Details: Accurate pH calculation is crucial for understanding titration curves, determining equivalence points, and analyzing buffer capacity in weak acid-strong base titrations.

4. Using the Calculator

Tips: Enter pK_a value, conjugate base concentration in mol/L, and weak acid concentration in mol/L. All values must be valid (pK_a ≥ 0, concentrations ≥ 0, weak acid > 0).

5. Frequently Asked Questions (FAQ)

Q1: What is the significance of the half-equivalence point?
A: At half-equivalence point, pH = pK_a and the solution has maximum buffer capacity.

Q2: When is the Henderson-Hasselbalch equation most accurate?
A: The equation is most accurate when the concentrations of [A⁻] and [HA] are within one order of magnitude of each other.

Q3: What are typical pK_a values for common weak acids?
A: Acetic acid: 4.76, Formic acid: 3.75, Carbonic acid: 6.35, Phosphoric acid: 2.14.

Q4: Can this equation be used for weak base-strong acid titrations?
A: Yes, with modification: pOH = pK_b + log([BH⁺]/[B]).

Q5: What are the limitations of the Henderson-Hasselbalch equation?
A: It assumes ideal behavior, neglects activity coefficients, and becomes less accurate at very low or very high concentrations.