Conjugate acid/base pairs solutions

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Write the proton transfer equilibria for the following acids in aqueous solution and identify the conjugate acid-base pairs in each one (a) H2S04 (b) C6H5NH3+. anilinium ion ... 

This equation is exact, but it can be simplified by applying one of the key features of buffer solutions. Any buffer solution contains both members of a conjugate acid-base pair as major species. In other words, both the weak acid and its conjugate base are present in relatively large amounts. As a result, the change to equilibrium, x, is small relative to each initial concentration, and the equilibrium concentrations are virtually the same as the initial leq linitial " = linitial... 

The pH of the buffer solution, both before and after adding the solid NaOH, is close to the p of the conjugate acid-base pair. Moreover, the pH increases when NaOH is added. The solution becomes more basic as a consequence of the added hydroxide anions. 

Because we know we are dealing with a buffer solution made from a specific conjugate acid-base pair, we can work directly with the buffer equation. We need to calculate the ratio of concentrations of conjugate base and acid that will produce a buffer solution of the desired pH. Then we use mole-mass-volume relationships to translate the ratio into actual quantities. 

A practical problem in solution preparation usually requires a different strategy than our standard seven-step procedure. The technician must first identify a suitable conjugate acid-base pair and decide what reagents to use. Then the concentrations must be calculated, using pH and total concentration. Finally, the technician must determine the amounts of starting materials. The technician needs a buffer at pH = 9.00. Of the buffer systems listed in Table 18-1. the combination of NH3 and NH4 has the proper pH range for the required buffer solution. 

C18-0053. From Table 18-1. select the best conjugate acid-base pairs for buffer solutions at pH 3.50 and 12.60. If you were going to add HCl solution as part of the buffer preparation, what other substance should you use in each case ... 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

Suppose a buffer solution of pH = 2.00 is needed. Suggest a conjugate acid-base pair for this solution, and calculate the ratio of the concentration of conjugate base to acid that is needed to prepare it. 

For example, to prepare a pH = 9 buffer solution, one would prepare a solution of ammonium chloride (refer to Table 5.1), and then add a solution of sodium hydroxide while stirring and monitoring the pH with a pH meter. The preparation is complete when the pH reaches 9. The required conjugate acid-base pair would be NH3 - NHj. Recipes for standard buffer solutions can be useful. Table 5.2 gives specific directions for preparing some popular buffer solutions. 

A problem exists with this procedure, however, in that at basic pH values, many metal ions precipitate as the hydroxide, e.g., Mg(OH)2, and thus would be lost to the analysis. This occurs with the magnesium in the water hardness procedure alluded to earlier. Luckily, a happy medium exists. At pH =10, the reaction of the metal ion with the predominant HY - and Y4- species (Figure 5.21) is shifted sufficiently to the right for the quantitative requirement to be fulfilled, while at the same time the solution is not basic enough for the magnesium ions to precipitate appreciably. Thus, all solutions in the reaction flask in the water hardness determination are buffered at pH = 10, meaning that a conjugate acid-base pair... 

Define buffer solution, conjugate acid, conjugate base, conjugate acid-base pair, buffer capacity, and buffer region. 

The solution is no longer HN02 and NaOH, but HN02 and N02 (a conjugate acid-base pair). 

The extent to which the pH of a solution is buffered against additions or removals of protons is measured by the solution s pH buffer capacity. This is defined as the amount of strong acid or base required to produce unit change in pH. The buffering depends on the transfer of protons between donors and acceptors, i.e. Bronsted acids and bases, which form conjugate acid-base pairs. The pH buffer capacity of a solution is calculated from the buffer capacities of the individual acid-base pairs present. 

Ka Kb = for a conjugate acid-base pair in aqueous solution. 

A most important relation exists between Ka and Kb of a conjugate acid-base pair in aqueous solution. We can derive this result with the acid HA and its conjugate base A. ... 

A solution contains 63 different conjugate acid-base pairs. Among them is acrylic acid and acrylate ion, with the equilibrium ratio [acrylate]/[acrylic acid] = 0.75. What is the pH of the solution ... 

For the following reaction in aqueous solution, identify the Bronsted-Lowry acids, bases, and conjugate acid-base pairs ... 

Write a balanced net ionic equation for the principal reaction in solutions of each of the following salts. In each case, identify the Bransted-Lowry acids and bases and the conjugate acid-base pairs. 

Which of the following conjugate acid-base pairs should you choose to prepare a buffer solution that has pH = 4. 50 Explain. 

An indicator is used to give a visual indication of the pH of a solution. Like a buffer, it is a conjugate acid-base pair however, indicators are used in such small amounts (a couple of drops) that the pH of the solution... 

Let s try another one. What is the pH of a buffer containing 0.25 M ammonia and 0.75 M ammonium chloride Well, the weak acid in this case is the ammonium ion. The chloride ion is a spectator to be ignored. Ammonia is a weak base, and the conjugate base of the ammonium ion. So, since this solution contains a weak conjugate acid-base pair, it is a buffer, and we can calculate the pH using the Henderson-Hasselbach equation. The Henderson-Hasselbach equation calls for the pKa of the acid so in this case, we need the pfor the ammonium ion. The pKh for ammonia is 4.74, so the pKa for the ammonium ion is 9.26. If we substitute this value and the values for the concentrations into the buffer equation, we find the pH of this solution is 8.78. 

PH is a measure of the concentration of H+ in a solution. An acid is a proton donor, a base is a proton acceptor. Ionization of an acid yields its conjugate base, and the two are termed a conjugate acid-base pair, for example acetic acid (CH3COOH) and acetate (CH3COO ). The pK of an acid is the pH at which it is half dissociated. The Henderson-Hasselbach equation expresses the relationship between pH, ptC and the ratio of acid to base, and can be used to calculate these values. 

A buffer solution is a solution that resists a change in pH after addition of small amounts of an acid or a base. Buffer solutions require the presence of an acid to neutralize an added base and also the presence of a base to neutralize an added acid. These two components present in the buffer also must not neutralize each other A conjugate acid-base pair is present in buffers to fulfill these requirements. 

The amount of acid or base that a buffer solution can neutralize before dramatic pH changes begins to occur is called its buffering capacity. Blood and seawater both contain several conjugate acid-base pairs to buffer the solution s pH and decrease the impact of acids and bases on living things. 

NH4F is dissolved in water. Which of the following are conjugate acid/base pairs present in the solution ... 

In the earlier work (Forster, 1951 Weller, 1952) when the principal experimental information involved fluorescence intensities, the most useful algebraic expressions were those relating relative quantum yields of the conjugate acid base pair to the solution acidity. In favourable cases such expressions were used to obtain rate constants for the forward and reverse reactions, and hence equilibrium constants, though it was always necessary to make allowance... 

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