Acid Equilibrium Constant (Ka)

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Jul 24, 2020, 6:28:07 PM

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May 21, 2016, 12:57:06 AM

K_{a} = \frac{[H_{3} O^{+}] [A^{-}]}{[H A]}

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

[H_{3} O^{+}] Concentration of free protons [H+] in solution.

$[H_3O^+] "Concentration of free protons [H+] in solution."$

[A^{-}] Concentration of conjugate base [A-] in solution.

$[A^-]" Concentration of conjugate base [A-] in solution."$

[H A] Concentration of weak acid [HA]

$[HA] "Concentration of weak acid [HA]"$

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.