Weak acids percent dissociation

Calculating the pH of Weak Acid Solutions The pH of a Mixture of Weak Acids Percent Dissociation ... 

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

The percent dissociation of a weak acid is the fraction of acid molecules that dissociate compared with the initial concentration of the acid, expressed as a percent. (Some chemists refer to percent dissociation as percent of dissociation.] The percent dissociation depends on the value of Ka for the acid, as well as the initial concentration of the weak acid. The following Sample Problems show how to solve problems that involve percent dissociation. 

The value of and the percent dissociation are reasonable for a weak acid. 

A quantitative measure of the degree of dissociation is given by the equilibrium constant for the acid or base. The higher the equilibrium constant is, the greater the percent dissociation of the acid or base. Therefore, a higher equilibrium constant means a stronger acid or base. Equilibrium constants, K and K, are listed for several com-mon weak acids and bases in Table 13.4. 

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 ... 

Iodic acid, HIO3, is a weak acid that undergoes only partial dissociation in water. If a 1.00 M solution of HIO3 has a density of 1.07 g/mL and a freezing point of —2.78°C, what percent of the HIO3 is dissociated ... 

In addition to Ka, another useful measure of the strength of a weak acid is the percent dissociation, defined as the concentration of the acid that dissociates divided by the initial concentration of the acid times 100% ... 

In general, the percent dissociation depends on the acid and increases with increasing value of Ka. For a given weak acid, the percent dissociation increases with increasing dilution, as shown in Figure 15.8. The 0.0100 M CH3C02H solution in Problem 15.14b, for example, has [H30+] = 4.2 X 10 4 M, and the percent dissociation is 4.2% ... 

Phenol (C5H5OH) is a weak acid used as a general disinfectant and in the manufacture of plastics. Calculate the pH and the concentrations of all species present (H30+,C6H50, C6H50H, and OH-) in a 0.10 M solution of phenol (Ka = 1.3 X 10-10). Also calculate the percent dissociation. 

Beginning with the equilibrium equation for the dissociation of a weak acid HA, show that the percent dissociation varies directly as the square root of Ka and inversely as the square root of the initial concentration of HA when the concentration of HA that dissociates is negligible compared with its initial concentration. 

Use data from the Testing pH activity (eChapter 15.7) to determine the of methylamine, CH3NH2. Determine the percent dissociation of methylamine at 0.10 M, 0.0010 M, and 1.0 X 10 5 M concentrations. What is the relationship between the percent dissociation and the pH for a weak base Would you expect the same relationship for a weak acid Explain. 

The following pictures represent solutions of a weak acid HA that may also contain the sodium salt NaA. Which solution has the highest pH, and which has the largest percent dissociation of HA (Na+ ions and solvent water molecules have been omitted for clarity.)... 

The real importance of the Henderson-Hasselbalch equation, particularly in biochemistry, is that it tells us how the pH affects the percent dissociation of a weak acid. Suppose, for example, that you have a solution containing the amino acid glycine, one of the molecules from which proteins are made, and that the pH of the solution is 2.00 pH units greater than the pKa of glycine ... 

The common-ion effect is the shift in the position of an equilibrium that occurs when a substance is added that provides more of an ion already involved in the equilibrium. An example is the decrease in percent dissociation of a weak acid on addition of its conjugate base. 

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. 

In this situation, if the hydrogen ion concentration increases (pH becomes lower), the reaction will be driven to the left by mass action (to the original condition), and the proportion of the drug in the nonionized form will increase and, hence, the number of lipid-soluble molecules. For example, if the pICa of a weak acid is 5.0 and it is placed in a medium of pH 4.0, 90 percent will be in the unionized form. Therefore, weak acids are preferentially absorbed in a relatively acidic environment. For a weak base, the equilibrium dissociation constant can be expressed as follows ... 

It is often useful to specify the amount of weak acid that has dissociated in achieving equilibrium in an aqueous solution. The percent dissociation is defined as follows ... 

For a given weak acid, the percent dissociation increases as the acid becomes more dilute. For example, the percent dissociation of acetic acid (HC2H302, Ka = 1.8 X 10-5) is significantly greater in a 0.10 M solution than in a 1.0 M solution. 

The effect of dilution on the percent dissociation and [H+] of a weak acid solution. 

The more dilute the weak acid solution, the greater the percent dissociation. 

Since Q is less than Ka, the system must adjust to the right to reach the new equilibrium position. Thus the percent dissociation increases as the acid becomes more dilute. This behavior is summarized in Fig. 7.5. In Example 7.4 we see how the percent dissociation can be used to calculate the Ka value for a weak acid. 

The small value for the percent dissociation clearly indicates that HC3H5O3 is a weak acid. Thus the major species in the solution are the undissociated acid and water ... 

Sketch two graphs (a) percent dissociation of weak acid HA versus initial concentration of HA ([HA]o), and (b) H+ concentration versus [HA]0. Explain both. 

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... 

Analytical chemists prepare solutions of known analytical concentrations, but the The analytical concentration repre-dissolved substances may partially or totally dissociate to give equilibrium con- sents the concentration of total dis-centrations of different species. Acetic acid, for example, is a weak acid that dis- solved substance, i.e., the sum of all sociates a few percent depending on the concentration. species of the substance in... 

Dilution of a weak acid increases its percent dissociation... 

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