Acids & Bases Problem Set

Acids & Bases Problem Set

Question 9: Acids & pK_a

Tutorial to help answer the question

Acids are defined as compounds with pK_a values below 7.0.

A. True

B. False

Tutorial

Acids

Acids are defined as compounds which can reversibly lose protons to the solution. The pH at which this occurs (the halfway point) is the pK_a. Strong acids such as HCl will give up protons even at very low pH (hence low pK_a) and weak acids will only give up protons if the pH is very high (i.e. the free proton concentration is very low). The pH at which this occurs can easily be above 7.

An important example is tyrosine, whose r-group is phenol; an acid with a pK_a ~ 10. Conversely, bases can have pK_a < 7.0. Can you think of an important biological BASE with a pK_a less than 7.0?

Most biological acids, however, are weaker acids than HCl. The major class of biological acids is carboxylic acids. Because the difference in electronegativity between oxygen and hydrogen in a carboxylic acid is not as dramatic as it is with HCl, the tendency of a carboxylic acid to give up its proton is much less than that of HCl. However, carboxylic acids dissociate more readily than water due to the presence of two electronegative oxygens.

An acid's tendency to dissociate is a function of the strength of the acid and the pH of the solution. Strong acids can still dissociate when the pH is low, whereas weak acids cannot. The convention is to identify the pH at which the acid is half dissociated (e.g. half is protonated and half is deprotonated). This pH value is defined as the pK_a of the acid in question. For example:

CH₃COO (acetic acid) CH₃COO^- (acetate ion) + H⁺ ; pK_a = 4.8;

meaning that, at pH = 4.8, half of the molecules are ionized (acetate) and half are not (acetic acid). The stronger the acid, the lower the pK_a.

Bases

In contrast to acids, bases are able to absorb protons from water and thus they are charged (+1) in the protonated form, and uncharged in the deprotonated form. The most common biological weak base is the amino group (R-NH₂).

Despite the differences between acids and bases, pK_a can also be used to quantitate the relative strength of bases. Notice that while the pK_a values for acids are generally less than 7, pK_a values for bases are usually greater than 7. For instance, ethanolamine has a pK_a of 9.5 and is thus half protonated (charge = +1/2) when the pH is 9.5 (Figure 4). An important exception to this rule of thumb is the amino acid histidine, which is a base but has a pK_a around 6.0.

The Biology Project
The University of Arizona
January 6, 1999
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