Diprotic solutions

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... 

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. 

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. 

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-]. 

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. 

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... 

At point A, the system is in the first buffer region and pH = pKa). Once all the acid H,P04 molecules have lost their first acidic protons, the system is at B and the primary species in solution are the diprotic conjugate base and sodium ion—we have a solution of NaH2P04(aq). Point B is the first stoichiometric point, and to reach it we need to supply 1 mol NaOH for each mole of H P04. 

Figure 11.14 shows the pH curve of a diprotic acid, such as oxalic acid, H2C204. There are two stoichiometric points (B and D) and two buffer regions (A and C). The major species present in solution at each point are indicated. Note that it takes twice as much base to reach the second stoichiometric point as it does to reach the first. 

FIGURE 11.14 The variation of the pH of the analyte solution during the titration ot a diprotic acid (oxalic acidl and the major species present in solution at the two stoichiometric points (B and D) and at points when half the titrant required to reach a stoichiometric point has been added (A and C). Compare this diagram with Fig. 10.20. The labels SP1 and SP2 denote the volumes of base required to reach the two stoichiometric points. 

Hydrogen sulfide dissolves in water to give a solution of hydrosulfuric acid that, as a result of its oxidation by dissolved air, slowly becomes cloudy as S8 molecules form and then coagulate. Hydrosulfuric acid is a weak diprotic acid and the parent acid of the hydrogen sulfides (which contain the HS ion) and the sulfides (which contain the S2 ion). The sulfides of the s-block elements are moderately soluble, whereas the sulfides of the heavy p- and d-block metals are generally very insoluble. 

Carbonated water contains carbonic acid, a diprotic acid that forms when carbon dioxide dissolves in water C02(g) +H2 0(/) H2 C03(c2 q) A typical carbonated beverage contains 0.050 M H2 CO3. Determine the concentrations of the ions present in this solution. 

C17-0095. Deeimine the concentrations of the Ionic species present in a 0.355 M solution of the diprotic acid H2 SO3. (See Table IM for K values.)... 

C17-0116. Oxalic acid, a diprotic carboxylic acid found in many plants, including rhubarb, is an effective stain remover. Consider the following diagram to be a small section of an aqueous solution of oxalic acid ... 

The titration of a diprotic acid, H2 A, can be divided into six regions that differ in the major species present in solution. Shown here is the titration curve for maleic acid (50.0 mL, 0.10 M) titrated with 1.0 M NaOH. 

At the beginning of the titration, the diprotic acid (represented by H2 A) and H2 O are the only major species in the solution. As we describe in Chapter 17, the hydronium ion concentration can be calculated from the... 

By recognizing species in solution and their dominant equilibrium, we can construct titration curves for other diprotic acids. Example shows how this is done for sulfurous acid. 

The starting pH of the solution is calculated using i and the initial molarity of the diprotic acid. We use the standard approach to a weak acid equilibrium ... 

The calculated curve shows the general features of the pH titration curve for a diprotic acid. The pH of the solution is acidic at the first stoichiometric point (major species = weak acid HA ) and basic at the second (major species =... 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

In a saturated solution, the three relevant equilibria for the case of a diprotic ampholyte are Eqs. (3.3) and (3.4), plus... 

Note in dilute solution, H2S04 is completely ionized and is considered to be a strong diprotic acid. 

A For an aqueous solution of a diprotic acid, the concentration of the divalent anion equals the second ionization constant ["OOCCH2COO" J = = 2.0x1 O 6 M. We organize... 

If one were to titrate a weak diprotic acid such as maleic acid with the known NaOH solution, the graph would show two separate inflections — representing the neutralization of each hydrogen — assuming the pKa values differ by 4 or more pK units (see Figure 2). The dissociation of a diprotic acid occurs in two separate steps ... 

The monoprotic model is appealing since it is very simple, realistic, and based on one less adjustable parameter than the diprotic model. The value of the parameter Ka can be found directly from the H+ concentration in the bulk of solution at Oq =0, since Ka = a + at this condition, according to Equation 6. Since the surface complexation models are already recognized as being underdetermined, any physically realistic model with fewer adjustable parameters is welcomed. 

Adipic acid is a diprotic acid that is used to manufacture nylon. Its formula can be abbreviated to H2Ad. The acid dissociation constants for adipic acid are = 3.71 x 10 and = 3.87 x 10 . What is the pH of a 0.085 mol/L solution of adipic acid ... 

O Oxalic acid, HOOCCOOH, is a weak diprotic acid that occurs naturally in some foods, including rhubarb. Calculate the pH of a solution of oxalic acid that is prepared by dissolving 2.5 g in 1.0 L of water. What is the concentration of hydrogen oxalate, HOOCCOO , in the solution ... 

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