Acid-ionization constant, Ka

Definition

The acid-ionization constant, $K_a$, quantifies the strength of a weak acid in solution. It is the equilibrium constant for the dissociation of an acid into its conjugate base and a proton.

5 Must Know Facts For Your Next Test

1. $K_a$ values are typically expressed in scientific notation due to their often small magnitude.
2. A larger $K_a$ value indicates a stronger weak acid because it signifies greater ionization in solution.
3. $K_a$ is calculated using the concentrations of the acid, its conjugate base, and hydrogen ions at equilibrium: $K_a = \frac{[H^+][A^-]}{[HA]}$.
4. The relationship between $pK_a$ and $K_a$ is given by $pK_a = -\log(K_a)$; hence, smaller $pK_a$ values correspond to stronger acids.
5. $K_w$, the ion-product constant for water, relates to $K_b$ (the base-ionization constant) and $K_a$: $K_w = K_a \cdot K_b$.

Review Questions

Related terms

Equilibrium Constant:

A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium for a reversible reaction.

$pH$: A measure of hydrogen ion concentration; a logarithmic scale used to specify the acidity or basicity of an aqueous solution.

$pKa$: The negative logarithm of the acid-ionization constant ($-\log(K_a)$); it provides insight into the strength of an acid.