what is the value of the acid dissociation constant for a weak acid?
how come the answer is less than one (1)?
Weak acids don't ionize 100%. In fact, that is the definition of a weak acid; i.e., strong acids ionize 100% while weak acids ionize less than 100%. If any acid ionizes less than 100% then the dissociation constant must be less than one because the numerator numbers will be less than the denominator numbers.
The value of the acid dissociation constant, also known as Ka, for a weak acid is typically less than one (1). The reason for this is rooted in the equilibrium expression used to calculate the value of Ka.
Let's consider the generic equation for the dissociation of a weak acid, HA:
HA ⇌ H+ + A-
In the forward reaction, the weak acid (HA) donates a proton (H+) to form the conjugate base (A-). The equilibrium expression for this reaction is given by:
Ka = [H+][A-] / [HA]
The numerator represents the concentration of the products (H+ and A-), while the denominator represents the concentration of the weak acid (HA). Since the concentration of the products is in the numerator, a higher value of Ka indicates a stronger acid.
When Ka is less than one, it means that the concentration of the products (H+ and A-) is relatively lower than the concentration of the reactant (HA). This suggests that the dissociation of the weak acid is incomplete, and a significant amount of the weak acid remains undissociated in solution.
In summary, the value of the acid dissociation constant (Ka) for a weak acid is less than one because the concentration of the products is relatively lower than the concentration of the reactant, indicating incomplete dissociation.
The value of the acid dissociation constant (Ka) for a weak acid is typically less than one (1). This is because the Ka expresses the degree of ionization for a weak acid in water, specifically the ratio of the concentration of the dissociated ions to the concentration of the undissociated acid molecules.
To calculate the acid dissociation constant, you need to know the initial concentration of the weak acid and the concentrations of the dissociated ions at equilibrium. By dividing the concentrations of the dissociated ions (such as H+ or H3O+) by the concentration of the undissociated acid, you obtain the value of Ka.
If the resulting Ka is less than one, it indicates that the weak acid is not completely ionized in solution. Instead, only a small proportion of the acid molecules dissociate into ions, and the majority remains as undissociated molecules. Consequently, the concentration of the dissociated ions is relatively low compared to the concentration of the undissociated acid, leading to a Ka value less than one.
For example, acetic acid (CH3COOH) is a weak acid with a Ka value of approximately 1.8 x 10^-5. This means that when acetic acid dissolves in water, only a small fraction of the acid molecules dissociate into hydrogen ions (H+) and acetate ions (CH3COO-), resulting in a low concentration of these ions compared to the concentration of undissociated acetic acid.
In summary, the value of the acid dissociation constant (Ka) for a weak acid is less than one because it reflects the limited extent of ionization of the acid in solution, with only a small proportion of acid molecules dissociating into ions.