Chapter 17, Problem 96GQ

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# A solution contains 0.10 M iodide ion, I−, and 0.10 M carbonate ion, CO32−. (a) If solid Pb(NO3)2 is slowly added to the solution, which salt will precipitate first, Pbl2 or PbCO3? (b) What will be the concentration of the first ion that precipitates (CO32− or I−) when the second, more soluble salt begins to precipitate?

(a)

Interpretation Introduction

Interpretation:

The salt of either PbI2 or PbCO3 precipitate first when solid Pb(NO3)2 is added to the solution containing lead and carbonate ions 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

When in the solution more than one insoluble salt having one common ion is present than the one which requires less amount of the common ion to exceed its solubility product value and precipitate will precipitate out first from the solution. This is known as preferential precipitation.

Given:

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

Solubility product constant Ksp for PbI2 is 9.8Ã—10âˆ’9.

Solubility product constant Ksp for PbCO3 is 7.4Ã—10âˆ’14.

The initial concentration of iodide ions present is 0.10Â M.

The initial concentration of carbonate ions present is 0.10Â M.

PbI2 dissociates as follows in water,

â€‚Â PbI2(s)â‡ŒPb2+(aq)+2Iâˆ’1(aq)

Ksp=[Pb2+][Iâˆ’1]2

Rearrange for [Pb2+] and substitute values.

[Pb2+]=Ksp[I]2=(9.8Ã—10âˆ’9)(0.10)2=9

(b)

Interpretation Introduction

Interpretation:

The concentration of first ion (CO32 or I1 ) that is present in the solution when second salt just begins to precipitate has to be calculated.

Concept introduction:

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

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