Solubility of sparingly soluble ionic compounds

It is convenient to define the solubility of ionic substances in units of g per 1000 g of saturated solution, or as the number of moles of substance needed to produce 1 dm of saturated solution. Molar solubility is defined as follows  

The molar solubility of a compound is Its concentration (moldm ) in a saturated solution at that temperature. 

Ionic compounds which are only slightly soluble in water are said to be sparingly soluble. Examples include silver chloride (Ag , Cl ) and barium sulfate (Ba , S04 ). Since such compounds are only slightly soluble, the volume of water used to dissolve a salt may be taken to be equal to the volume of the solution. (For example, if we dissolve 0.0010 g of silver chloride in 1000 cm of water, we may safely assume that the volume of the solution is also 1000 cm. ) This approximation does 

One reminder - although many ionic substances are only sparingly soluble in water, any ionic substance that does dissolve completely dissociates into separate ions. 

The solubility of silver chloride is 1.8 x 10 gof AgCI per 1000 g of saturated AgCI solution at 25°C. (a) Express this solubility in mol of AgCI per dm of AgCI solution, (b) What is the concentration of Ag+(aq) and Cl (aq) (in moldm ) in a saturated solution of AgCI (Assume that 1 cm of solution has a mass of exactly 1 g at 25 °C.) 

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You will determine the solubilities of sparingly soluble ionic compounds. 

CD-ROM Simulation "Exploration of Solubility of Sparingly Soluble Ionic Compounds. 

As more ions enter the solution, the rate of the reverse change, recrystallisation, increases. Eventually, the rate of recrystallisation becomes equal to the rate of dissolving. As you know, when the forward rate and the backward rate of a process are equal, the system is at equilibrium. Because the reactants and the products are in different phases, the reaction is said to have reached heterogeneous equilibrium. For solubility systems of sparingly soluble ionic compounds, equilibrium exists between the solid ionic compound and its dissociated ions in solution. 

You can use the relationship between the ion product expression and the solubility product expression to predict whether a precipitate will form in a given system. One common system involves mixing solutions of two soluble ionic compounds, which react to form an ionic compound with a very low solubility. If Qsp > Kp. based on the initial concentrations of the ions in solution, the sparingly soluble compound will form a precipitate. 

Many biological and environmental processes involve the dissolution or precipitation of a sparingly soluble ionic compound. Tooth decay, for example, begins when tooth enamel, composed of the mineral hydroxyapatite, Cas PO OH,... 

P The solubility of a sparingly 1 soluble ionic compound is increased on addition of H30 + if the anion of the compound is the conjugate base of a weak acid. 

All precipitates show some kind of ageing, which causes a decrease in solubility. Therefore, they are usually filtered after some time of standing or after heating. Sparingly soluble ionic compounds in contact with a solution exhibit continuous recrystallization. Both processes, ageing and recrystallization, can be investigated successfully by application of radiotracers. 

The product of and the concentration of the undissolved solid creates a new constant called the solubility product constant, The solubility product constant is an equilihrium constant for the dissolving of a sparingly soluble ionic compound in water. The solubility product constant expression is... 

How can you use the solubility product constant to calculate the solubility of a sparingly soluble ionic compound ... 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

The difference in concentration of ions in the two half-cells of a concentration cell accounts for the observed EceU- It also provides a basis for determining values for sparingly soluble ionic compounds. Consider the following concentration cell. 

The basis of these methods is the linear dependence of the absorbance of a solution on the concentration of the various absorbing solutes (Beer s law). Therefore, fundamental requisites are the adherence of the solutes to Beer s law and the constant absorptivity of each one of these species with changing solvent composition. When these requirements are met, the experimentally determined ratio of the concentrations of the ionized to the neutral species (say Q-/Cah) at different pH values leads to thermodynamic pKs (after the appropriate corrections for ionic strength effects). These methods are particularly valuable for the study of sparingly soluble compounds. 

Neutral organometallic compounds are mostly sparingly soluble only (or after extensive period of sonication) in ionic liquids. 

The compound Q QH2, quinhydrone, is sparingly soluble in water, producing equal concentrations of Q, quinone, and QH2, hydroquinone. Using the values of the mean ionic activity... 

The solubility products of selected compounds are listed in Table 11.4. Solubility product expressions can be used only for sparingly soluble salts. They cannot be used (without modification) for very soluble salts (like sodium chloride) because the concentration of ions is so high that the ions influence each other and the effective concentration of ions is lower than their concentration in mol dm . The errors in using similar equations for slightly soluble ionic substances (such as calcium hydroxide) are smaller, but are still significant in accurate work. (Now try Exercise 1 IE.)... 

Solubility product Solubility product constant, iQp, reflects the relationship between dissolved species and precipitated species. Each ionic compound has its own solubility limit, which is the maximum amount of the compound that can remain in solution. IQp is commonly used in solubility calculations to determine the precipitation potential of mineral salts. Certain combinations of cations and anions form sparingly soluble salts in water, and scaling in RO/NF may occur when the salts are concentrated beyond their solubility limits. See Table 6.10. 

Compounds are often regarded as being soluble or insoluble. However, many ionic compounds, such as silver chloride, are sparingly soluble in water. When increasing quantities of a sparingly soluble ionic solid are added to water, a saturated solution is eventually formed. There is a dynamic equilibrium between the undissolved salt and its dissolved ions ... 

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