`K_a = ([H_3O^+][A^-])/([HA])`

Enter a value for all fields

The Equilibrium Constant for an acid (K_a) calculator computes Ka based on the concentration of free floating protons [H₃O⁺], conjugate base [A^-] and the weak acid added to the solution [HA].

INSTRUCTION: Choose units and enter the following:

• [H₃O⁺] This is the concentration of free floating protons
• [A^-] This is the concentration of the conjugate base
• [HA] This is the concentration of weak acid added to the solution

Equilibrium Constant for an acid (K_a) :  The calculator returns the constant in moles per liter.

The Math / Science

The formula for the Equilibrium Constant for an acid (K_a) is:

  Ka = [H₃O⁺]•[A^-] / [HA]

where:

• Ka is the Equilibrium Constant for an acid 
• [H₃O⁺] is the concentration of free floating protons
• [A^-] is the concentration of the conjugate base
• [HA] is the concentration of weak acid added to the solution

The Equilibrium Constant for an acid (K_a) is found using the equation K_a = [H₃O⁺][A^-] / [HA], with [H₃O⁺] representing the free floating protons, [A^-] the conjugate base, and [HA] the original weak acid added to solution. K_a refers to the dissociation of a weak acid in solution, and uses the Brönsted-Lowry definition of an acid, as opposed to Arrhenius. The inputs are the molarity of the hydronium [H3O+], the conjugate base [A-], and the acid [HA]. While this version of the equation uses molarity, a more complete definition uses molar activity instead of molarity. Molar activity accounts for both the molarity and non-ideal conditions. Molar activity is a unitless term, therefore the acid dissociation constant is unitless.

Uses

K_a is useful for biochemists and organic chemists among others, but can also prove useful in any situation involving or requiring pH or pOH. The base equilibrium constant K_a can be used in the following constant formula, K_w = K_a * K_b, to solve for K_b, which in turn can be used to find pOH. Since K_w is a constant with a value of 1.00 x 10¹⁴, if either K_b or K_a is known, then the other equilibrium constant can be calculated. Similarly, K_a can be used to find the pH of a solution.

References

Whitten, et al. "Chemistry" 10th Edition. Pp. 734

Wikipedia: Equilibrium Constant

Chemistry.osu

Ausetute, ChemTeam

Supplemental Material

Wikipedia: Equilibrium Constant

Chemistry.osu

Ausetute, ChemTeam

See also:

Base Equilibrium Constant (for the Spanish site click here)