Ionization, percent

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

For any acid, the concentration of acid that ionizes equals the concentration of H (aq) that forms, assuming that H2O autoionization is negligible. Thus, the percent 

As calculated in Sample Exercise 16.10, a 0.10 M solution of formic add (HCOOH) contains 4.2 X 10 M Calculate the percentage of the acid that is ionized. 

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. 

Knowing the value of K and the initial concentration of a weak acid, we can calculate the concentration of H (aq) in a solution of the acid. Let s calculate the pH at 25 °C of a 0.30 M solution of acetic acid (CH3COOH), the weak acid responsible for the characteristic odor and acidity of vinegar. 

When we first encountered acids [M4 Section 4.1], we learned that a strong acid is one that ionizes completely. Weak acids ionize only partially. The degree to which a weak acid ionizes is a measure of its strength, just as the magnitude of its ionization constant (A ) is a measure of its strength. A quantitative measure of the degree of ionization is percent ionization, which, for a weak, monoprotic acid (HA) is calculated as follows  

We have learned previously how to use coUigative properties to determine the percent ionization of a weak electrolyte, such as a weak acid [ M4 Section 13.6], Now that we have an understanding of equilibrium and of ionization constants, we can predict the percent ionization of a weak acid. Moreover, we will discover that the percent ionization is not constant for a given weak acid, but in fact depends on the acid s concentration. 

Consider a 0.10 Ab solution of benzoic add, for which the ionization constant (/fa) is 6.5 X 10 . Using the procedure described in Sample Problem 16.12, we can determine the equilibrium concentrations of benzoic acid, H, and the benzoate ion. 

Solving for x gives 0.0025 M. Therefore, at equihbrium, [CeHsCOOH] = 0.097 Ab and [H ] = [CgHsCOO ] = 0.0025 Ab. The percent ionization of benzoic acid at this concentration is 

Now consider what happens when we dilute this equilibrium mixture by adding enough water to double the volume. The concentrations of all three species are cut in half [CsHsCOOH] = 0.049 Ab, [H ] = 0.0013 Ab, and [CsHsCOO ] = 0.0013 Ab. If we plug these new concentrations 

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

As you might expect, percent ionization at a given concentration is directly related to fCa. Ibuprofen (K = 2.5 X 10-5), a weaker acid than aspirin (fCa = 3.6 X 10-4), should be only 3.6% ionized at 0.020 M compared with 12% for aspirin. 

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... 

Determine the percent ionization for an aqueous solution of HE that is 25 mM. 

To determine percent ionization, we need to know the equilibrium concentration of hydronium ions. This requires an equilibrium calculation, for which we follow the seven-step method. We need to set up the appropriate equilibrium expression and solve for [H3 O, after which we can use Equation to... 

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. 

C17-0064. Determine the percent ionization of the solution of hydrazoic acid in Problem. ... 

C17-0068. Determine the percent ionization of a solution of acetic acid that is 0.75 M (see Appendix E for K values). 

C17-0129. Prepare a graph of the percent ionization of aqueous acetic acid as a function of the logarithm of its... 

Further implications of this equation are that at one pH unit below or above the pKa, an ionizable group will be 91 percent unionized (protonated) or 91 percent ionized (unprotonated), respectively. Two pH units below or above translate to a 99 percent unionized or 99 percent ionized state. 

As you can see, the percent ionization decreases as the solution concentration increases. 

If the molarity of acetic acid is doubled, we expect a lower initial pH (more H30+(aq) in solution) and a lower percent ionization as the concentration increases. Therefore (b) (the diagram in the center) best represents the conditions ( (2)1/2 times greater). 

Information other than solubility that can help select the suitable mode of chromatography, whether or not the sample is ionic. In this regard, the dissociation constant of the compound is of great value because with appropriate adjustment of pH, one can select a desirable percent ionization of the compound of interest, remembering when pH = pXa, the compound is 50% ionized. 

Do the following approximate percents ionization fit your laboratory results glacial acetic acid, 0.1% 6.0M acetic acid, 0.2% 1.0/V7 acetic acid, 0.4% 0.1 M acetic acid, 1.3% Explain. 

Acids and bases are a big part of organic chemistry, but the emphasis is much different from what you may be familiar with from your general chemistry course. Most of the attention in general chemistry is given to numerical calculations pH, percent ionization, buffer problems, and so on. Some of this returns in organic chemistry, but mostly we are concerned with the roles that acids and bases play as reactants, products, and catalysts in chemical reactions. We ll start by reviewing some general ideas about acids and bases. 

The ionization constant for formic acid, HC02H, is 1.77 x 10 4. What is the percent ionization of a 0.00100 M solution of formic acid ... 

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%. 

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