   Chapter 17, Problem 95GQ

Chapter
Section
Textbook Problem

In principle, the ions Ba2+ and Ca2+ can be separated by the difference in solubility of their fluorides, BaF2 and CaF2. If you have a solution that is 0.10 M in both Ba2+ and Ca2+, CaF2, will begin to precipitate first as fluoride ion is added slowly to the solution. (a) What concentration of fluoride ion will precipitate the maximum amount of Ca2+ ion without precipitating BaF2? (b) What concentration of Ca2+ remains in solution when BaF2 just begins to precipitate?

(a)

Interpretation Introduction

Interpretation:

Concentration of Fluoride ions that will precipitate the maximum amount of calcium ions without precipitating BaF2 has to be calculated.

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

Precipitation of salt occurs when the ionic product exceeds the solubility product. Using Ksp calculate the concentration of fluoride ions. Then precipitation of BaF2 will take place only when the concentration of [F1] exceeds the value calculated using Ksp value.

Concentration of Fluoride ions that will precipitate the maximum amount of calcium ions is calculated below.

Given:

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

Solubility product constant Ksp for BaF2 is 1.8×107.

The concentration of barium ions present is 0.10 M.

BaF2 dissociates as follows in water,

BaF2(s)Ba2+(aq)+2F1(aq)

The expression for Ksp,

Ksp=[Ba2+][F1]2

Rearrange for [F1] and substitute values

(b)

Interpretation Introduction

Interpretation:

Concentration of the calcium ions that is present in the solution when BaF2 just begins to precipitate has to be calculated.

Concept introduction:

When in the solution more than one insoluble salt having one common ion  is present than the one which require 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.

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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