Meaning and mechanism
Weak HA dissociates in water, forming H⁺ and A⁻ ions. A larger Ka value indicates a greater tendency to dissociate under the same conditions.
At the same concentration, strong acids and weak acids exhibit different hydrogen ion concentrations. While diluting a weak acid may increase its dissociation ratio, the hydrogen ion concentration itself can decrease.
Relationships and conditions
For a monoprotic weak acid, neglecting water’s autoionization gives Ka=x²/(C−x), where x is the hydrogen-ion concentration produced by dissociation. pH depends on the actual H⁺ concentration, rather than the acid’s name alone.
Only when x is much smaller than C may C−x be approximated by C. Very dilute solutions and acids that dissociate in several stages require additional equilibria.
What to observe in the simulation
The strength of an acid is its tendency to dissociate, and the concentration is the amount of acid in a unit volume. It is classified as pH = −log₁₀[H⁺], α = [H⁺]/C.
25℃ is the ideal concentration balance of monoprotic acid. For weak acids, use the exact solution x²+Kax−KaC=0. Activity and diversity assets are excluded. Auto-ionization of water is only ignored for [H⁺] ≥ 10⁻⁶ mol/L (sufficiently greater than 10⁻⁷). The entire input range satisfies this condition. Ka=1.8×10⁻⁵ is an example value for acetic acid. Particle ratios are rounded to the nearest 100.
Key relationships and symbols
Ka = x²/(C−x) · x = 2KaC/(√(Ka²+4KaC)+Ka)
| Symbol | Meaning | Unit |
|---|---|---|
| Ka | Acid dissociation constant | At standard conditions |
| C, x | Formal acid concentration and amount dissociated | mol/L |
Worked example
How do pH and dissociation change when the same weak acid is diluted?
If diluted at the same Ka, [H⁺] decreases and pH increases. At the same time, the dissociated ratio α of all acids increases. Ka is constant at the same temperature.
Common misconceptions
- A weak acid is not necessarily a dilute acid.