It is a quite accepted classification of strong acids in water , to state that they completely dissociate. For a strong acid we can see that the concentration of undissociated acid is nearly zero. Strictly speaking, in aqueous solutions the acidity constants cannot be measured, since the hydronium ion is the strongest acid in this medium.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. At what pKa value do we consider an acid strong? Ask Question. Therefore, our above equation simplifies to:. Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic. Calculate percent dissociation for weak acids from their K a values and a given concentration.
Calculate the percent dissociation of a weak acid in a [latex]0. We set up our equation as follows:. However, because the acid dissociates only to a very slight extent, we can assume x is small. The above equation simplifies to the following:. As we would expect for a weak acid, the percent dissociation is quite small. For example, with a problem involving the percent dissociation of a 0. An ICE chart is used to calculate the relevant concentrations.
Privacy Policy. Skip to main content. Acids and Bases. Search for:. Strength of Acids Strong Acids In water, strong acids completely dissociate into free protons and their conjugate base. Learning Objectives Calculate pH for solutions of strong acids. The use of pK a values allows us to express the acidity of common compounds and functional groups on a numerical scale of about —10 very strong acid to 50 not acidic at all.
Alkenes and alkanes, which are not acidic at all, have pK a values above The lower the pK a value, the stronger the acid. It is important to realize that pK a is not the same thing as pH: pK a is an inherent property of a compound or functional group, while pH is the measure of the hydronium ion concentration in a particular aqueous solution:.
Any particular acid will always have the same pK a assuming that we are talking about an aqueous solution at room temperature but different aqueous solutions of the acid could have different pH values, depending on how much acid is added to how much water. Our table of pK a values will also allow us to compare the strengths of different bases by comparing the pK a values of their conjugate acids.
The key idea to remember is this: the stronger the conjugate acid, the weaker the conjugate base. Sulfuric acid is the strongest acid on our list with a pK a value of —10, so HSO 4 - is the weakest conjugate base. Here is where your familiarity with organic functional groups will come in very handy. What, for example, is the pK a of cyclohexanol?
Notice in this example that we need to evaluate the potential acidity at four different locations on the molecule. Aldehyde and aromatic protons are not at all acidic pK a values are above 40 — not on our table. The two protons on the carbon next to the carbonyl are slightly acidic, with pK a values around according to the table. The most acidic proton is on the phenol group, so if the compound were to be reacted with a single molar equivalent of strong base, this is the proton that would be donated first.
Carboxylic acids containing -COOH , such as acetic and lactic acids, normally have a Ka constant of about 10 -3 to 10 Consequently, expressing acidity in terms of the Ka constant alone can be inconvenient and not very intuitive. Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.
In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3. Another important point is the relationship between pH and the pKa of an acid.
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