   Chapter 17, Problem 104IL

Chapter
Section
Textbook Problem

You will often work with salts of Fe3+, Pb2+, and Al3+ in the laboratory. (All are found in nature, and all are important economically.) If you have a solution containing these three ions, each at a concentration of 0.10 M, what is the order in which their hydroxides precipitate as aqueous NaOH is slowly added to the solution?

Interpretation Introduction

Interpretation:

The order in which hydroxides of Al3+, Fe3+ and Pb2+ will precipitate out from a solution containing these three ions as aqueous NaOH is added to the solution has to be predicted.

Concept introduction: The solubility of a salt is defined as the maximum amount of salt that can be dissolved in definite amount of solvent. It is expressed in moles per liter or grams per liter. Solubility in terms of moles per liter is called molar solubility and is defined as the number of moles of solute (salt) dissolved in per liter of solution.

Solubility product constant Ksp is an equilibrium constant and is defined as the product of the equilibrium concentration of the ions of the salt raised to the power of their coefficients in the balanced chemical equation.

The expression for Ksp of a salt is given as,

AxBy(s)xAy+(aq)+yBx(aq)Ksp=[Ay+]x[Bx]y

Explanation

Refer to the Appendix J in the textbook for the value of Ksp.

Solubility product constant Ksp for Pb(OH)2 is, for Fe(OH)3 is 1.1×1036 and for Al(OH)3 is 1.8×1033.

The concentration of each ion in the solution is 0.1 M.

In water Fe(OH)3 dissociates as,

Fe(OH)3(s)Fe3+(aq)+3OH1(aq)

Ksp=[Fe3+][OH1]3[OH1]=Ksp[Fe3+]3

Substitute the values.

[OH1]=1.1×10360.13=2.22×1012M

In water Pb(OH)2 dissociates as,

Pb(OH)2(s)Pb2+(aq)+2OH1(aq)

Ksp=[Pb2+][OH1]2[OH1]=Ksp[Pb2+]

Substitute the values

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