Acid-base equilibrium problems

Let s begin with a series of acid-base equilibria problems. 

A wide variety of acid—base equilibria problems can be solved using the relationships between pH, hydrogen ion concentration and hydroxide ion concentration in conjunction with the ionic product constant of water. 

This is a quantitative acid-base equilibrium problem, so we use the seven-step method. 

The key to solving acid-base equilibrium problems is to think about the chemistry—that is, to consider the possible proton-transfer reactions that can take place between Bronsted-Lowry acids and bases. 

No all-purpose rules can be given for acid-base equilibrium problems. Skill increases with experience ... 

Recognize acid or base strengths from Ka or pKa values and use these to predict the position of an acid-base equilibrium. (Problems 4.23.4.34.4.35,4.36.4.37, and 4.41)... 

Having a conceptual understanding of the effect is a good starting point, but we still need to be able to understand the quantitative relationships between the different components in the equilibrium mixture. In this section, we will see how to deal with the common-ion effect in acid-base equilibrium problems. You will find that these problems are very similar to the weak acid problems earlier in the chapter. 

In this chapter we have encountered many different situations involving aqueous solutions of acids and bases, and in the next chapter we will encounter still more. In solving for the equilibrium concentrations in these aqueous solutions, you may be tempted to create a pigeonhole for each possible situation and to memorize the procedures necessary to deal with each particular situation. This approach is just not practical and usually leads to frustration Too many pigeonholes are required, because there seems to be an infinite number of cases. But you can handle any case successfully by taking a systematic, patient, and thoughtful approach. When analyzing an acid-base equilibrium problem, do not ask yourself how a memorized solution can be used to solve the problem. Instead, ask yourself this question What are the major species in the solution, and how does each behave chemically ... 

The most important part of doing a complicated acid-base equilibrium problem is the analysis you do at the beginning of a problem ... 

Because acid-base reactions in solution generally are so rapid, we can concern ourselves primarily with the determination of species concentrations at equilibrium. Usually, we desire to know [H+], [OH ], and the concentration of the acid and its conjugate base that result when an acid or a base is added to water. As we shall see later in this text, acid-base equilibrium calculations are of central importance in the chemistry of natural waters and in water and wastewater treatment processes. The purpose of this section is to develop a general approach to the solution of acid-base equilibrium problems and to apply this approach to a variety of situations involving strong and weak acids and bases. 

Checking the assumption that 2S 10 we find 2S = 7 x 10 which is not IQ- The assumption is not valid. We will therefore proceed as we did in the solution of acid-base equilibrium problems and make the opposite assumption, that is, assume that 2S lO-". Then... 

As an example of an acid-base equilibrium problem, consider water in equilibrium with atmospheric carbon dioxide. The value of [COj (aq)] in water at 25°C in equilibrium with air that is 390 parts per million COj (close to the concentration of this gas in the atmosphere) is 1.277 X 10 mol/L. The carbon dioxide dissociates partially in water to produce equal concentrations of H+ and HCO3" ... 

It is well known that the rates of all azo coupling reactions in aqueous or partly aqueous solutions are highly dependent on acidity. Conant and Peterson (1930) made the first quantitative investigation of this problem. They demonstrated that the rate of coupling of a series of naphtholsulfonic acids is proportional to [OH-] in the range pH 4.50-9.15. They concluded that the substitution proper is preceded by an acid-base equilibrium in one of the two reactants, which was assumed to be the equilibrium between the diazohydroxide and the diazonium ion, in other words, that the reacting equilibrium forms are the undissociated naphthol and the diazohydroxide. 

Problems that involve the concentrations of ions formed in aqueous solutions are considered to be equilibrium problems. The steps for solving acid and base equilibrium problems are similar to the steps you learned in Chapter 7 for solving equilibrium problems. 

The steps that you will use to solve acid and base equilibrium problems will vary depending on the problem. Below are a few general... 

Many of these are substantially non-nucleophilic and unlikely to effect the rate or course of the reaction, although this should always be checked. References 29 to 31 relate some problems in the use of some of these buffers. Occasionally, one of the reactants being used in excess may possess buffer capacity and this obviates the necessity for added buffer. The situation will often arise in the study of complex ion-ligand interactions when either reactant may be involved in an acid-base equilibrium. 

One thing is clear there can be no acid-base equilibrium between states of different multiplicities thus it is correct to consider only the pK of the singlet state , or the pA of the triplet state . However, the question of the protolytic equilibrium between an mr singlet and a tttt or charge transfer (CT) singlet remains open. This problem is illustrated in Figure 4.48 for the case of 4-hydroxybenzophenone, in which there is a reversal in the order of mr and CT states between the acid and base forms. Excitation of the protonated molecule in ethanol for example leads to the ground state deprotonated form, but the detailed mechanism of this process is not known. 

FIGURE 7.4 Of the 16 chemistry topics examined (1-16) on the final exam, overall the POGIL students had more correct responses to the same topics than their L-I counterparts. Some topics did not appear on all the POGIL exams. Asterisks indicate topics that were asked every semester and compared to the L-I group. The topics included a solution problem (1), Lewis structures (2), chiral center identification (3), salt dissociation (4), neutralization (5), acid-base equilibrium (6), radioactive half-life (7), isomerism (8), ionic compounds (9), biological condensation/hydrolysis (10), intermolecular forces (11), functional group identification (12), salt formation (13), biomolecule identification (14), LeChatelier s principle (15), and physical/chemical property (16). 

First in-situ infrared investigations of phosphoric acid adsorption on platinum and gold were performed in HCIO4 as base electrolyte [138]. More recently, spectroscopic data in alkaline solutions were reported [37, 158]. However, not enough attention was paid in these studies to the problem of acid-base equilibrium displacements in solution and to the overlapping of solution and surface features which make the interpretation of spectra very difficult. Results on the adsorption of phosphate species on polycrystalline platinum at pH 2.8 (79% H2PO4 and 21 % H3PO4) are shown in Fig. 58a [146]. 

Obviously, all acid-base equilibrium constants depend on the pH scale used. It is possible to convert approximately an equilibrium constant determined in one scale to that of another scale. The problem of different definitions of equilibrium constants needs attention when applying an infinite dilution scale complex formation constant,—for example, for CuC03(aq)—in a seawater medium. 

Outline the procedure for the exact treatment of acid-base equilibrium and use it to find the pH of a very dilute solution of a weak acid or base (Section 15.8, Problems 69-70). 

The acid-base equilibrium constants of the beta-blockers atenolol, oxprenolol, timolol, and labetalol were determined by automated potentiometric titrations. The pKg values were obtained in water-rich or water methanol medium (20% MeOH) to obviate the solubility problems associated with the compounds. The initial estimates of pKa values were obtained from Gran s method and then, were refined by the NYTIT and ZETA versions of the LETAGROP computer program. The resultant values were 9.4 (/ = 0.1 M KCI, 20% methanol) for atenolol, 9.6 (/ = 0.1 M KCI) for oxprenolol, 9.4 (/ = 0.1 M KCI, 20% methanol) for timolol and 7.4 and 9.4 (/ = 0.1 M KCI) for labetalol. The potentiometric method was found to be accurate and easily applicable. The operational criteria for applying the methodology are indicated. 

Before studying this chapter, review Section 5.4 on limiting quantities problems and, before Section 10.2, Acid-Base Equilibrium, review net ionic equations from the textbook. 

Solutions Manual (0-13-147882-6) The Solutions Manual, prepared by Jan W. Simek of California Polytechnic State University, contains complete solutions to all the problems. The Solutions Manual also gives helpful hints on how to approach each kind of problem. This supplement is a useful aid for any student, and it is particularly valuable for students who feel they understand the material but need more help with problem solving. Appendix 1 of the Solutions Manual summarizes the lUPAC system of nomenclature. Appendix 2 reviews and demonstrates how acidity varies with structure in organic molecules, and how one can predict the direction of an acid-base equilibrium. Brief answers to many of the in-chapter problems are given at the back of this book. These answers are sufficient for a student on the right track, but they are of limited use to one who is having difficulty working the problems. 

The equilibrium parameters of Lux acid-base reactions in ionic media (solubility products of oxides and acid-base equilibrium constants) are essentially affected by the acidic properties of the molten alkaline halide mixtures, i.e., they are dependent on the constituent cation acidities. Therefore, one should consider the reverse problem - the estimation of the basicity indices of ionic melts on the basis of the calculated equilibrium constants. 

Analyze We are asked to determine the pH at the equivalence point of the titration of a weak acid with a strong base. Because tire neutralization of a weak acid produces the corresponding conjugate base, we ejqsect the pH to be basic at the equivalence point. Plan We should first determine how many moles of acetic acid there are initially. This win teU us how many moles of acetate ion there will be in solution at the equivalence point. We then must determine the final volume of the resulting solution, and the concentration of acetate ion. From this point this is simply a weak-base equilibrium problem like those in Section 16.7. 

Refer to the steps for solving weak acid equilibrium problems. Use the same systematic approach for weak base equilibrium problems. 

Often we know the value of Kgp for a compound and are asked to calculate the compound s molar solubility. The procedure for solving such a problem is essentially identical to the p ocedure for solving weak acid or weak base equilibrium problems ... 

Sodium ar chloride comprise the bulk of the electrolytes in plasma and interstitial fluid. Sodium constitutes 90 % of the total base of the plasma, the normal concentration being 140 meq. per liter. The normal concentration of chloride is 104 meq. per liter. The sodium ion plays an important role in the maintenance of acid-base equilibrium and in the maintenance of osmotic pressure, which depends largely on total base. Cations in blood, other than sodium, are calcium, potassium, and magnesium anions, other than chloride, are bicarbonate, protein, and small amounts of organic acid. The pH is usually regulated by the relative amounts of chloride and bicarbonate. Acidosis and alkalosis are encountered in many diseases of man, but these problems belong in the fleld of clinical medicine rather than nutrition and will not be discussed here. 

If we make the same x is small approximation that we make for weak acid or weak base equilibrium problems, we can consider the equilibrium concentrations of HA and A to be essentially identical to the initial concentrations of HA and A (see step 4 of Example 16.1). Therefore, to determine [H30 ] for any buffer solution, we multiply by the ratio of the... 

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