Calculate Ka From Molarity

Weak Acid Ka Equation:

\[ K_a = \frac{[H^+]^2}{C - [H^+]} \quad \text{where} \quad [H^+] = 10^{-pH} \]

Acid Dissociation Constant (Ka):

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1. What is the Ka Calculation?

The acid dissociation constant (Ka) calculation determines the strength of a weak acid from its pH and concentration. Ka quantifies the extent to which an acid dissociates in solution, with larger values indicating stronger acids.

2. How Does the Calculator Work?

The calculator uses the weak acid Ka equation:

\[ K_a = \frac{[H^+]^2}{C - [H^+]} \quad \text{where} \quad [H^+] = 10^{-pH} \]

Where:

\( K_a \) — Acid dissociation constant (M)
\( [H^+] \) — Hydrogen ion concentration (M)
\( C \) — Total acid concentration (M)
\( pH \) — Negative logarithm of hydrogen ion concentration

Explanation: This formula assumes the acid is monoprotic and that [H+] comes primarily from the acid's dissociation, which is valid for weak acids where [H+] is much smaller than C.

3. Importance of Ka Calculation

Details: Ka values are essential for understanding acid strength, predicting buffer behavior, calculating pH of acid solutions, and designing chemical processes involving acid-base reactions.

4. Using the Calculator

Tips: Enter pH value (0-14) and acid concentration in molarity (M). Ensure pH is accurate and concentration is greater than the calculated [H+] concentration for valid results.

5. Frequently Asked Questions (FAQ)

Q1: What is the range of typical Ka values?
A: Ka values range from about 10⁻¹⁰ for very weak acids to 10¹⁰ for very strong acids, with pKa (=-logKa) being more commonly reported.

Q2: When is this calculation valid?
A: This calculation is valid for weak monoprotic acids where [H+] << C and the acid concentration is significantly greater than the autoionization of water (10⁻⁷ M).

Q3: What if [H+] is close to or greater than C?
A: This indicates either measurement error, a strong acid, or very high dilution. The calculation becomes invalid as it violates the weak acid assumption.

Q4: How accurate is this method?
A: For true weak acids with accurate pH measurements, this method provides good estimates. For more precise work, consider activity coefficients and temperature effects.

Q5: Can this be used for polyprotic acids?
A: No, this formula is for monoprotic acids only. Polyprotic acids require more complex calculations accounting for multiple dissociation steps.