Acidity diprotic

Strong acid Weak acid Diprotic acid Oxyacids Organic acids Carboxyl group Amphoteric substance Ionization of water Ion-product constant... 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Bronsted-Lowry acids and bases donate or accept protons. A Lewis acid or base accepts or donates a pair of electrons. Key Terms Bronsted-Lowry acid Bronsted-Lowry base Bronsted-Lowry acid-base reaction 1 lie polyprotic acid diprotic acid triprotic acid Ul IvO Lewis base Lewis acid-base reaction... 

Nucleotides are phosphoric acid esters of nucleosides Those derived from adenosine of which adenosine 5 monophosphate (AMP) is but one example are especially promi nent AMP is a weak diprotic acid with s for ionization of 3 8 and 6 2 respectively In aqueous solution at pH 7 both OH groups of the P(0)(0H)2 unit are ionized... 

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

We treat HP04 as the intermediate form of a diprotic weak acid... 

Multiprotic weak acids can be used to prepare buffers at as many different pH s as there are acidic protons. For example, a diprotic weak acid can be used to prepare buffers at two pH s and a triprotic weak acid can be used to prepare three different buffers. The Henderson-Hasselbalch equation applies in each case. Thus, buffers of malonic acid (pKai = 2.85 and = 5.70) can be prepared for which... 

Construct ladder diagrams for the following diprotic weak acids (H2L), and estimate the pH of 0.10 M solutions of H2L, HL , and Using the systematic approach, calculate the pH of each of these solutions. 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

Another situation in which an inflection point may be missing or difficult to detect occurs when the analyte is a multiprotic weak acid or base whose successive dissociation constants are similar in magnitude. To see why this is true let s consider the titration of a diprotic weak acid, H2A, with NaOH. During the titration the following two reactions occur. 

Thus, if we titrate a monoprotic weak acid with a strong base, the EW and FW are identical. If the weak acid is diprotic, however, and we titrate to its second equivalence point, the FW will be twice as large as the EW. 

If the weak acid is monoprotic, then the FW must be 58.78 g/mol, eliminating ascorbic acid as a possibility. If the weak acid is diprotic, then the FW may be either 58.78 g/mol or 117.6 g/mol, depending on whether the titration was to the first or second equivalence point. Succinic acid, with a formula weight of 118.1 g/mol is a possibility, but malonic acid is not. If the analyte is a triprotic weak acid, then its FW must be 58.78 g/mol, 117.6 g/mol, or 176.3 g/mol. None of these values is close to the formula weight for citric acid, eliminating it as a possibility. Only succinic acid provides a possible match. 

Tartaric acid, H2C4H4O6, is a diprotic weak acid with a pK i of 3.0 and a pK 2 of 4.4. Suppose you have a sample of impure tartaric acid (%purity > 80) and that you plan to determine its purity by titrating with a solution of 0.1 M NaOH using a visual indicator to signal the end point. Describe how you would carry out the analysis, paying particular attention to how much sample you would use, the desired pH range over which you would like the visual indicator to operate, and how you would calculate the %w/w tartaric acid. 

Certain weak acids are polyprotic they contain more than one ionizable hydrogen atom. Such acids ionize in steps, with a separate equilibrium constant for each step. Oxalic acid, a weak organic acid sometimes used to remove bloodstains, is diprotic ... 

The conclusions reached in Example 13.9 are ordinarily valid for any weak diprotic acid, H2B. 

Ascorbic acid, H2C6H606, also known as vitamin C, is present in many citrus fruits. It is a diprotic acid with the following values = 7.9 X 105 K = 1-6 X 10 12. What is the pH of a 0.63 M solution of ascorbic acid Estimate [HC6H606-] and [C4H6062-]. 

To treat acid-base equilibria involving zwitterions, it is convenient to consider the cation stable at low pH to be a diprotic add (analogous to H2C03), which ionizes in two steps. Using the symbols C+, Z, and A- to stand for the cation, zwitterion, and anion, respectively, we have... 

When a polyprotic acid is dissolved in water, the various hydrogen atoms undergo ionisation to different extents. For a diprotic acid H2A, the primary and secondary dissociations can be represented by the equations ... 

It can be shown that the pH at the first equivalence point for a diprotic acid is given by... 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Neutralisation reactions. The equivalent of an acid is that mass of it which contains 1.008 (more accurately 1.0078) g of replaceable hydrogen. The equivalent of a monoprotic acid, such as hydrochloric, hydrobromic, hydriodic, nitric, perchloric, or acetic acid, is identical with the mole. A normal solution of a monoprotic acid will therefore contain 1 mole per L of solution. The equivalent of a diprotic acid (e.g. sulphuric or oxalic acid), or of a triprotic acid (e.g. phosphoric( V) acid) is likewise one-half or one-third respectively, of the mole. 

L.30 You have been given a sample of an unknown diprotic acid (a) Analysis of the acid shows that a 10.0-g sample contains 0.224 g of hydrogen, 2.67 g of carbon and the rest oxygen. Determine the empirical formula of the acid. 

STRATEGY Verify that Eq. 14 can be used by checking that S 5i> K JKal and S 5i> fC,. If so, we use Eq. 14 to determine the pH of the salts of the diprotic conjugate base (H,A ") of a triprotic acid (H SA) and the monoprotic conjugate base (HA ) of a diprotic acid (H2A). However, when the solure is a salt of an anion that has lost two protons, such as HP042-, we must adjust the expression to use the appropriate neighboring pkas. 

Self-Test 10.17B Protonated glycine (+NH3CH2COOH) is a diprotic amino acid with fCal = 4.5 X 10 3 and K.a = 1.7 X 10 10. Calculate the concentrations of all solute species in 0.50. vi NH3CH2COOHCI(aq). 

We have found expressions for the fractions, /, of species in a solution of carbonic acid. They are easily generalized to any diprotic acid H2A ... 

For each of the following polyprotic acids, state which species (H2A, HA, or A2 of the diprotic acid or H3A, H,A, HA2, or A for triprotic acids) you expect to be the form present in highest concentration in aqueous solution at pH = 5.0 (a) tartaric acid (b) hydrosulfuric acid (c) phosphoric acid. 

Like sulfuric acid, a certain diprotic acid, H,A, is a strong acid in its first deprotonation and a weak acid in its second deprotonation. A solution that is 0.020 M H,A(aq) has a pH of 1.66. What is the value of fCa2 for this acid ... 

Suppose we are titrating the triprotic acid H P04 with a solution of NaOH. The experimentally determined pH curve is shown in Fig. 11.13. Notice that there are three stoichiometric points (B, D, and F) and three buffer regions (A, C, and E). In pH calculations for these systems, we assume that, as we add the hydroxide solution, initially NaOH reacts completely with the acid to form the diprotic conjugate base... 

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