The Equilibrium Constant for a base (Kb) is found using the equation Kb = [BH+][OH-] / [B], with [BH+] representing the protonated base, [OH-] the hydroxide ions, and [B] the original base added to solution. Kb refers to the dissociation of a base in solution, and uses the Brönsted-Lowry definition of a base, as opposed to Arrhenius. The inputs are the concentration of the protonated base [BH+], the hydroxide ions [OH-], and the unchanged base [B]. 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 base dissociation constant is unitless.
Kb is useful for biochemists, but can also prove useful in any situation involving or requiring pH or pOH. The base equilibrium constant Kb can be used in the following constant formula, Kw = Ka * Kb, to solve for Ka, which in turn can be used to find pH. Since Kw is a constant with a value of 1.00 x 1014, if either Kb or Ka is known, then the other equilibrium constant can be calculated. Similarly, Kb can be used to find the pOH of a solution.
Whitten, et al. "Chemistry" 10th Edition. Pp. 734
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