Concentration calculating with acid-ionization constants

Diprotic and polyprotic acids undergo successive ionizations, losing one proton at a time (W Section 4.3], and each ionization has a ATa associated with it Ionization constants fCT a diprotic acid are designated and We write a separate equUibrium exja ession for each ionization, and we may need two or more equilibrium expressions to calculate the concentrations of species in solution at equilibrium. For carbonic acid (H2CO3), for example, we write... 

The pH of the acetic acid solution is higher (it is less acidic) because acetic acid only partially ionizes. Calculating the [H30 ] formed by the ionization of a weak acid requires solving an equilibrium problem similar to those in Chapter 14. Consider, for example, a 0.10 M solution of the generic weak acid HA with an acid ionization constant K. Since we can ignore the contribution of the autoionization of water, we only have to determine the concentration of H30 formed by the following equilibrium ... 

In these equations, HA symbolizes a weak acid and A symbolizes the anion of the weak acid. The calculations are beyond our scope. However, we can correlate the value of the equilibrium constant for a weak acid ionization, Ka, with the position of the titration curve. The weaker the acid, the smaller the IQ and the higher the level of the initial steady increase. Figure 5.2 shows a family of curves representing several acids at a concentration of 0.10 M titrated with a strong base. The curves for HC1 and acetic acid (represented as HAc) are shown, as well as two curves for two acids even weaker than acetic acid. (The IQ s are indicated.)... 

Consider a solution of a monoprotic weak acid with acidity constant Ka. Calculate the minimum concentration, C, for which the percent ionization is less than 10%. 

A volume of 50.00 mL of a weak acid of unknown concentration is titrated with a 0.1000 M solution of NaOH. The equivalence point is reached after 39.30 mL of NaOH solution has been added. At the half-equivalence point (19.65 mL) the pH is 4.85. Calculate the original concentration of the acid and its ionization constant K. ... 

Calculate the concentration of CSH4O4 (a) in a 0.010 M solution of H2CgH404, (b) in a solution which is 0.010 M with respect to H2C8H4O4 and 0.020 M with respect to HCl. The ionization constants for H2C8H4O4, phthalic acid, are... 

In contrast to the previous problem, in which strong acid or base was added, we can assume that the pH is not changed by addition of the phosphoric acid, especially since this solution was previously well buffered with respect to the very small amount of H3PO4 added. Then, if [H ] is fixed, the ratio of two of the desired concentrations can be calculated from each of the ionization constant equations. 

The calculation for the ionization of a weak base parallels that used with weak acids in Examples 17.2 and 17.3 you write the equation, make a table of concentrations (Step 1), set up the equilibrium-constant equation for Kj, (Step 2), and solve for x = [OH ] (Step 3). Assume the self-ionization of water can be neglected. You obtain [HsO ], and then the pH, by solving K = [H30 ][OH ]. 

The first proton of sulfuric acid is completely ionized, but the second proton is only partially dissociated, with an acidity constant of 1.2 X 10 -. Calculate the hydrogen ion concentration in a 0.0100 M H2SO4 solution. 

Figure 11.33 Ionization of p-nitrophenol in the presence of sodium dodecyl sulphate. The fractional ionization, a, was calculated from the absorbance of the p-nitrophenolate ion at 400 nm. The total p-nitrophenol concentration was 1 x 10 moll The solvent was 0.004 m sodium phosphate buffer, or 0.004 m glycylglycine buffer for the higher pH values, adjusted to the pH indicated and to a constant ionic strength of 0.1 m with NaCl. The solid lines are standard titration curves for the dissociation of a monobasic acid. The NaLS concentrations in g ml" were a, 0 b, 0.0144 c, 0.0288 d, 0.0576. From Herries et al [218].

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