Calculate acid dissociation constant Ka, convert Ka to pKa, or estimate buffer pH from pH, pKa, and [A−]/[HA] ratios for solutions.

Dilute-solution approximation: concentrations represent activities relative to 1 mol/L. Use equilibrium concentrations for one conjugate acid–base pair at a fixed temperature.

Equilibrium concentration, not the amount initially mixed.

Equilibrium concentration, not the amount initially mixed.

Equilibrium concentration, not the amount initially mixed.

Acid Dissociation Constant Formula

The following dilute-solution approximation uses equilibrium concentrations in mol/L for one conjugate acid–base pair. Thermodynamic Ka uses activities; concentrated or coupled equilibria need a fuller model. Select the value to calculate in Solve for. 

Ka = A × H / HA

Variables:

  • Ka is the dimensionless acid dissociation constant, approximated here using equilibrium concentrations relative to 1 mol/L
  • A is the concentration of conjugate base of acid 
  • H is the concentration of hydrogen ions 
  • HA is the concentration of chemical species (HA) 

To calculate the acid dissociation constant, multiply the concentration of the conjugate base of acid by the concentration of hydrogen ions, then divide by the concentration of the chemical species.

How to Calculate Acid Dissociation Constant?

The following steps outline how to calculate the Acid Dissociation Constant.

  1. First, determine the concentration of conjugate base of acid.
  2. Next, determine the concentration of hydrogen ions. 
  3. Next, determine the concentration of chemical species (HA). 
  4. Next, gather the formula from above = Ka = A*H/HA.
  5. Finally, calculate the Acid Dissociation Constant.
  6. After inserting the variables and calculating the result, check your answer with the calculator above. 

Example Problem:

Use the following variables as an example problem to test your knowledge.

concentration of conjugate base of acid = 0.001 mol/L

concentration of hydrogen ions = 0.00001 mol/L

concentration of chemical species (HA) = 0.01 mol/L

Ka = A*H/HA = (0.001 × 0.00001) / 0.01 = 0.000001