Weak acid percent ionization

The purpose of calculating the percent ionization for three acetic acid solutions was to confirm the very important point made on page 748 For a weak acid, percent ionization increases with increasing dilution (Fig. 16-7). For very dilute solutions, the calculation of percent ionization is more complicated. (See Are You Wondering 16-2 on page 756.)... 

As a family, carboxylic acids are weak acids that ionize only slightly in water. As aqueous solutions, typical carboxylic acids ionize to the extent of only one percent or less. 

Acetic acid is a weak acid, which ionizes only partially (a few percent) ... 

C, = 1.8 x 10-5). Like any weak acid, the percent ionization of acetic acid is inversely related to its molar concentration. 

Percent ionization also depends on the concentration of weak acid, increasing as the acid is diluted (Figure 13.8). 

In the box below, which has a volume of 0.50 L, the symbol represents 0.10 mol of a weak acid, HB. The symbol 9 represents 0.10 mol of the conjugate base, B . Hydronium ions and water molecules are not shown. What is the percent ionization of the acid ... 

Show by calculation that when the concentration of a weak acid decreases by a factor of 10, its percent ionization increases by a factor of 10l/2. 

As Figure 17-6 indicates, only a fraction of weak acid molecules undergoes proton transfer to form hydronium ions. Equation defines the percent ionization of any acid solution to be the percentage of the initial concentration... 

When we know the pH and initial concentration of a weak acid solution, we can calculate the percent ionization quickly. Eor the solution of HE in Example, we have pH = 1.92, from which [H3 O ]gq= 1.2x 10 M and we have [HFUal - 0-25 M. Substituting, we find... 

The resuits of Exampie and Extra Practice Exercise illustrate an important feature of percent ionization. As the concentration of a weak acid increases, the percent ionization decreases. The concentration of hydronium ion decreases as a weak acid is diluted, but it decreases by a smaller fraction than the dilution factor. This behavior of weak acids played an important role in the development of the ionic view of aqueous solutions, as described in our Chemical Milestones Box. 

C17-0104. Determine the percent ionization of the weak acid solution in Problem. ... 

As pure compounds, acids are covalent. When placed in water, they react with the water to form ions it is said that they ionize. If they react 100% with the water, they are said to be strong acids. The seven common strong acids are listed in Table 7-3. All the rest are weak that is, the rest ionize only a few percent, and largely stay in their covalent forms. Both strong and weak acids react 100% with metal hydroxides. All soluble metal hydroxides are ionic in water. 

Acids are classified as strong or weak, depending upon their degree of ionization in water. A weak acid ionizes in water reversibly to form HjO ions. A weak acid is a weak electrolyte, and its aqueous solution does not conduct electricity well. The dissociation reaction occurs to a very small extent usually, fewer than 1 percent of the HA molecules are ionized. The ionization of a weak acid is shown as follows ... 

M of a weak acid solution (HA) ionizes 0.2 percent in water. Find its acid dissociation constant (Kg) and its pH value ... 

Partially ionized substances are written in the ionic form only if the extent of ionization is appreciable (about 20 percent or more). Water, which is ionized to the extent of less than one part in a hundred million, is written as H20 (or, if more convenient, HOH). Strong acids, like HC1 and HNO3, may be written in the ionized form, but weak acids, like nitrous, acetic, and sulfurous acids, are written in the molecular form (HNO2, HC2H3O2 and H2SO3). Ammonia, a weak base, is written NH3. Sodium hydroxide, a strong base, is written in the ionized form when in aqueous solution. 

Consider a solution of a monoprotic weak acid with acidity constant Ka. Calculate the minimum concentration, C, for which the percent ionization is less than 10%. 

Figure 2.11. Percent of ionogenic (ionizable) species present for weak acids and bases when solution pH is 2 units above or below the acid dissociation constant.

This equation essentially describes the relationship between pH and the degree of ionization of weak acids and bases. When applied to drugs, the equation tells us that when pH equals the apparent equilibrium dissociation constant of the drug (pKJ, 50 percent of the drug will be in the unionized form and 50 percent will be in the ionized form (i.e., log[base/acid] = 0 and antilog of 0 = 1, or unity). Application of the Henderson-Hasselbalch equation can, therefore, allow one to mathematically determine the exact proportion of ionized and nonionized species of a drug in a particular body compartment if the pKa of the drug and the pH of the local environment are known. 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Bronsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Bronsted-Lowry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 percent ionized in solution. That means that when acetic acid, HC2H302, is placed in water, the reaction looks like ... 

Notice the diversity in structure of lhe.se proton dunurs. They include the classical hydrochloric acid (reaction a). Ihc weakly acidic dihydrogen phosphate anion (reaction b). the ammonium cation as is found in ammonium chloride (reaction c), the carboxylic acetic acid (reaction d). Ihc cnolic form of phenobarbital (reaction e), Ihe carboxylic acid moiety of indomelhacin (reaction j), Ihc imidc of saccharin (reaction g). and the prolonaied amine of ephedrine (reaction h). Because all are proton donors, they mu.st be treated as acids when calculating the pH uf a solution or percent ionization of the drug. At the same lime, as nuted below, there are important differences in the pharmaceutical properties of ephedrine hydrochloride (an acid salt of an amine) and lho.se of indomelhacin. phenobarbital. or saccharin. 

Effect of pH for lonizable Organic Solutes The distribution of weak acids and bases between organic and aqueous phases is dramatically affected by the pH of the aqueous phase relative to the plQ of the solute. As discussed earher, the pK is the pH at which 50 percent of the solute is in the ionized state. (See Dissociative Extraction in Commercial Extraction Schemes. ) For a weak organic acid (RCOOH) that dissociates into RCOO and H , the overall partition ratio for extraction into an organic phase depends upon the extent of dissociation such that... 

Lactic acid, C2H4(OF[)COOH, is found in sour milk. It is also formed in muscles during intense physical activity and is responsible for the pain felt during strenuous exercise. It is a weak monoprotic acid and therefore a weak electrolyte. The freezing point of a 0.0100 m aqueous solution of lactic acid is —0.0206°C. Calculate (a) the / value and (b) the percent ionization in the solution. 

A weak acid ionizes only slightly in solution, perhaps only to a few percent, and in solution between the molecular acid is in equilibrium with its ions. There are thousands of known weak acids and five of the more common ones are listed below with the equation for their equilibrium in solution. The equilibrium constant for the ionization of weak acids is symbolized Ka, and is called the acid-ionization constant. 

FI6URE 15.3 The extent of ionization of (a) a strong acid that undergoes 100 percent ionization, (bj a weak acid, and (c) a very weak acid. 

FIGURE 15.4 Dependence of percent ionization on initial concentration of acid. Note that at very low concentrations, all acids (weak and strong) are almost completely ionized. 

The extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percent ionization (Figure 15.4). In qualitative terms, when an acid is diluted, the number of particles (nonionized acid molecules plus ions) per unit volume is reduced. According to Le Chateher s principle (see Section 14.5), to counteract this stress (that is, the dilution), the equilibrium shifts from nonionized acid to H+ and its conjugate base to produce more particles (ions). The dependence of percent ionization on initial concentration can be illustrated by the HF case discussed on page 608 ... 

Percent ionization is another measure of the strength of acids. The more dilute a solution of a weak acid, the greater the percent ionization of the acid. 

By how much should a 0.100 M solution of a weak acid HA be diluted in order to double its percent ionization Assume C > 100i a. 

The acetic acid, which is only a few percent ionized, depending on the concentration, is neutralized to water and an equivalent amount of the salt, sodium acetate. Before the titration is started, we have 0.1 MHOAc, and the pH is calculated as described for weak acids in Chapter 7. Table 8.2 summarizes the equations... 

The strength of an acid or base is determined by the extent of its ionization in aqueous solution. Strong acids, such as hydrochloric acid, are 100 percent ionized in aqueous solution, whereas weak acids, such as acetic acid, are less than 5 percent ionized. Experimentally, the extent of ionization is determined by measuring the electrical conductance of solutions. Strong acids and bases are strong electrolytes, and weak acids and bases are weak... 

Note that the extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percent ionization (see Figure 15.4 of the text). 

We have seen that the magnitude of indicates the strength of a weak acid. Another measure of acid strength is percent ionization, defined as... 

Analyze We are given the molar concentration of an aqueous solution of weak acid and the equilibrium concentration of H (aq) and asked to determine the percent ionization of the acid. 

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