Calculate Ka From pH

Ka Calculation Formula:

\[ Ka = \frac{[H^+]^2}{C - [H^+]} \approx \text{for weak acid} \]

Acid Dissociation Constant (Ka):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Ka Calculation From pH?

The acid dissociation constant (Ka) calculation from pH allows determination of the strength of a weak acid using pH measurements and known concentration. This method is particularly useful for weak acids where [H+] is much smaller than the initial concentration.

2. How Does the Calculator Work?

The calculator uses the Ka calculation formula:

\[ Ka = \frac{[H^+]^2}{C - [H^+]} \]

Where:

\( [H^+] \) — Hydrogen ion concentration = \( 10^{-pH} \) (M)
\( C \) — Initial concentration of weak acid (M)
\( Ka \) — Acid dissociation constant (M)

Explanation: This approximation assumes the acid is weak enough that [H+] is significantly less than C, and that the contribution from water autoionization is negligible.

3. Importance of Ka Calculation

Details: Ka values are fundamental in acid-base chemistry, helping determine acid strength, predict reaction outcomes, and understand buffer systems. Accurate Ka determination is essential for pharmaceutical formulations, environmental chemistry, and biochemical applications.

4. Using the Calculator

Tips: Enter pH value (0-14) and acid concentration in molarity (M). Ensure pH measurements are accurate and concentration values are precise for reliable results.

5. Frequently Asked Questions (FAQ)

Q1: When is this approximation valid?
A: This approximation works best for weak acids where [H+] < 5% of C. For stronger acids, more exact calculations are needed.

Q2: What are typical Ka values for common acids?
A: Strong acids have Ka > 1, weak acids have Ka < 1. For example, acetic acid Ka ≈ 1.8×10⁻⁵, while hydrochloric acid Ka ≈ 10⁷.

Q3: Can this method be used for polyprotic acids?
A: For polyprotic acids, this method gives Ka₁. Subsequent Ka values require more complex calculations considering multiple equilibria.

Q4: What affects the accuracy of Ka determination?
A: Temperature, ionic strength, measurement precision of pH, and purity of the acid solution all affect accuracy.

Q5: How does temperature affect Ka values?
A: Ka values are temperature-dependent. Most determinations are done at 25°C for standardization and comparison purposes.