The Solubility-Product Constant, Ksp

As with any other equilibrium, the extent to which this dissolution reaction occurs is expressed by the magnitude of the equilibrium constant. Because this equilibrium equation describes the dissolution of a solid, the equilibrium constant indicates how soluble the solid is in water and is referred to as the solubility-product constant (or simply the solubility product). It is denoted K p, where sp stands for solubility product The equilibrium-constant expression for the equilibrium between a solid and an aqueous solution of its component ions (Ksp) is written according to the rules that apply to any other equilibrium-constant expression. Remember, however, that soUds do not appear in the equihbrium-constant expressions for heterogeneous equihbrium. ooo (Section 15.4) Thus, the solubihty-product expression for BaS04, which is based on Equation 17.15, is 

In general, the solubility product Ksp of a compound equals the product of the concentration of the ions involved in the equilibrium, each raised to the power of its coefficient in the equilibrium equation. The coefficient for each ion in the equilibrium equation also equals its subscript in the compound s chemical formula. 

The values of Ksp at 25 °C for many ionic solids are tabulated in Appendix D. The 

Write the expression for the solubihty-product constant for Cap2, and look up the corresponding Ksp value in Appendix D. 

Analyze We are asked to write an equilibrium-constant expression for the process by which Cap2 dissolves in water. 

This equilibrium is known as a heterogeneous equilibrium, which is to say that it consists of substances that are in different states (phases). If you recall from Chapter 13, the equilibrium constant expressions for such equilibria do not contain the concentrations of liquids or solids. The equilibrium expression for this reaction will describe the degree to which the solid dissolves in solution, which is another way to say the degree to which it is soluble. And since the solid is not shown in the equilibrium expression, the equilibrium constant will express the product of the concentrations of the dissolved solute ions. For this reason, the equilibrium constant is referred to as the solubility-product constant. For this equilibrium, the solubility-product constant, Ksp is  

One important consideration is the absence of the solid from the expression. Even though the solid (Pbl2) is not shown in the equilibrium expression, it must be present in the solution in order for the system to be at equilibrium. In problems involving Ksp, you will usually encounter the information that some undissolved solid remains, which is your cue to know that it is okay to use the Ksp expression. 

The value of Ksp for Pbl2 at 25°C is 9.8 X 10 9. Small numbers like this Eire a good indicator that the material is not very soluble. 

It is important to make the distinction between solubility and the solubility product constant. Solubility refers to the amount of a substance that can dissolve to form a saturated solution. The solubility of a substance is usually expressed in grams solute per liter of solution (g/L). Sometimes, however, it is described in moles solute per liter of solution, in which case it is referred to as the molar solubility of the substance. At any given temperature, the solubility-product constant, Ksp, for a solute remains constant. The solubility of a solute is not necessarily constant because it is affected by the concentrations of other dissolved substances. In the next section we will look at some factors that affect the solubility of a solute. 

One type of calculation that you may be asked to perform is the determination of the concentration of an ion in solution when given the Ksp, or you may be asked to calculate the Ksp when given the concentration of an ion in solution. The two sample problems that follow will demonstrate these techniques. 

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One way to establish equilibrium between a slightly soluble solid and its ions in solution is to stir the solid with water to form a saturated solution. As you might expect, the solubility of the solid, s, in moles per liter, is related to the solubility product constant, Ksp. In the case of barium sulfate dissolving in water we have... 

The equilibrium constant expression associated with systems of slightly soluble salts is the solubility product constant, Ksp. It is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation. It contains no denominator since the concentration of a solid is, by convention, 1, and for this reason it does not appear in the equilibrium constant expression. The Ksp expression for the PbS04 system is ... 

The solubility product constant, Ksp, is the equilibrium constant expression for sparingly soluble salts. It is the product of the ionic concentration of the ions, each raised to the power of the coefficient of the balanced chemical equation. 

From the above expression the solubility product constant Ksp may be designated as ... 

In Chapter 9, as in most of Unit 4, you learned about equilibrium reactions. In this section, you analyzed precipitation reactions. You mainly examined double-displacement reactions—reactions in which two soluble ionic compounds react to form a precipitate. You used the solubility product constant, Ksp, to predict whether or not a precipitate would form for given concentrations of ions. In Unit 5, you will learn about a class of reactions that will probably be new to you. You will see how these reactions interconvert chemical and electrical energy. 

Write the equation for the slight dissolution of the insoluble compound and calculate the AG°(reaction) using Equation (6) from Chapter 2. Once the AG°(reaction) value is obtained, the K value is calculated using Equation (8) also from Chapter 2. Keeping in mind that the concentration (activity) of the insoluble compound is defined as 1, it is recognized that the K value for the dissolution of the insoluble compound is the solubility product constant, Ksp. Alternatively, the Ksp value may be available from compilations of such values as presented in Lange s Handbook of Chemistry or the CRC Handbook of Chemistry and Physics. ... 

The solubility of an ionic compound can be described quantitatively by a value called the solubility product constant, Ksp. For the general solubility process AaBi, a An+ +b Bm, Ksp = [A, +HBm"]6. The brackets refer to concentrations in moles per liter. 

The solubility product constant (Ksp) of EDTA was determined by adjusting the pH of an aqueous solution to a low value using nitric acid, and leaving the system to reach equilibrium overnight at room temperature. The precipitate was filtered off, dried at 105°C, and weighed to determine the amount of solubilized material. Alternatively, the precipitate was analyzed by complexometric titration, using standardized 0.05 M Zn(II) solution and xylenol orange as indicator [12]. The estimated value of the solubility product is 10 24 66 (pKsp = 24.66). 

In this chapter, we will extend the concepts of equilibrium that have been discussed in previous chapters. In Chapter 10 we discussed the concept of equilibrium in relation to saturated solutions in which an equilibrium was established between solvated ions and undissolved solute. In Chapter 11 we discussed the solubility of different salts when we looked at the formation of precipitates. In this chapter you will see the connection between these two ideas with the introduction of the solubility product constant, Ksp, which is a quantitative means of describing solubility equilibria. This measure helps to predict and explain the precipitation of different salts from solution. You will also see how the common-ion effect, temperature, and pH affect solubility. 

The equilibrium between ionic solutes and their constituent ions can be summarized using the equilibrium constant known as the solubility product constant, Ksp. 

The equilibrium constant for the dissolution of a slightly soluble salt is the solubility product constant Ksp. 

Calculate the solubility-product constant, Ksp, of lead(II) chloride, PbCl2, which has a solubility of 1.00 g/100.0 g H2O at a temperature other than 25 °C. 

The Solubility-Product Constant (Ksp) Calculations Involving Ksp The Effect of a Common Ion The Effect of pH Qsp rs. K ... 

The solubility of a substance can change considerably in response to a number of factors. For example, the solubilities of hydroxide salts, like Mg(OH)2, are dependent upon the pH of the solution. The solubility is also affected by concentrations of other ions in solution, especially common ions. In other words, the numeric value of the solubility of a given solute does change as the other species in solution change. In contrast, the solubility-product constant, Ksp, has only one value for a given solute at any specific temperature. A Figure 17.16 summarizes the relationships among various expressions of solubility and K p. 

To what reaction does the solubility product constant, Ksp, refer Table 16.1 lists /Tsp values for several ionic solids. For any of these ionic compounds, you should be able to calculate the solubility. What is the solubility of a salt, and what procedures do you follow to calculate the solubility of a salt How would you calculate the Ksp value for a salt given the solubility ... 

The Solubility-Product Constant, Ksp The solubility-product constant of an ionic compound is the equilibrium constant for the chemical equation that describes the dissolving of the compound. 

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