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When a strong acid is dissolved in water, it reacts with it to form hydronium ion (H 3 O +). [2] An example of this would be the following reaction, where "HA" is the strong acid: HA + H 2 O → A − + H 3 O + Any acid that is stronger than H 3 O + reacts with H 2 O to form H 3 O +. Therefore, no acid stronger than H 3 O + exists in H 2 O.
Acetic acid is an example of a weak acid. The pH of the neutralized solution resulting from HA + OH − → H 2 O + A −. is not close to 7, as with a strong acid, but depends on the acid dissociation constant, K a, of the acid. The pH at the end-point or equivalence point in a titration may be calculated as follows.
For example, acetic acid is a weak acid which has a = 1.75 x 10 −5. Its conjugate base is the acetate ion with K b = 10 −14 /K a = 5.7 x 10 −10 (from the relationship K a × K b = 10 −14), which certainly does not correspond to a strong base. The conjugate of a weak acid is often a weak base and vice versa.
An example of an alkalimetric titration involving a strong acid is as follows: H 2 SO 4 + 2 OH − → SO 4 2-+ 2 H 2 O. In this case, the strong acid (H 2 SO 4) is neutralized by the base until all of the acid has reacted. This allows the viewer to calculate the concentration of the acid from the volume of the standard base that is used.
However, for weak acids, a quadratic equation must be solved, and for weak bases, a cubic equation is required. In general, a set of non-linear simultaneous equations must be solved. Water itself is a weak acid and a weak base, so its dissociation must be taken into account at high pH and low solute concentration (see Amphoterism).
If one reagent is a weak acid or base and the other is a strong acid or base, the titration curve is irregular and the pH shifts less with small additions of titrant near the equivalence point. For example, the titration curve for the titration between oxalic acid (a weak acid) and sodium hydroxide (a strong base) is
A weak acid may be defined as an acid with pK a greater than about −2. An acid with pK a = −2 would be 99 % dissociated at pH 0, that is, in a 1 M HCl solution. Any acid with a pK a less than about −2 is said to be a strong acid. Strong acids are said to be fully dissociated.
In water, measurable pK a values range from about −2 for a strong acid to about 12 for a very weak acid (or strong base). A buffer solution of a desired pH can be prepared as a mixture of a weak acid and its conjugate base. In practice, the mixture can be created by dissolving the acid in water, and adding the requisite amount of strong acid ...