   Chapter 16, Problem 6RQ

Chapter
Section
Textbook Problem

# Mixtures of metal ions in aqueous solution can sometimes be separated by selective precipitation. What is selective precipitation? If a solution contained 0.10 M Mg2+, 0.10 M Ca2+, and 0. 10 M Ba2+, how could addition of NaF be used to separate the cations out of solution—that is, what would precipitate first, then second, then third? How could addition of K3PO4 be used to separate out the cations in a solution that is 1.0 M Ag+, 1.0 M Pb2+, and 1.0 M Sr2+?

Interpretation Introduction

Interpretation: The definition of selective precipitation is to be stated. The concentration of Mg2+,Ca2+ and Ba2+ is given. The order of appearance of precipitate is to be stated when NaF is added to these ions. The concentration of Ag+,Pb2+ and Sr2+ is given. The order of appearance of precipitate is to be stated when K3PO4 is added to these ions.

Concept introduction: Separation of mixtures of metal ions in aqueous solution using a reagent is known as selective precipitation.

Explanation

Explanation

To determine: The definition of selective precipitation; the order of appearance of precipitate when NaF is added to Mg2+,Ca2+ and Ba2+ ions; the order of appearance of precipitate is to be stated when K3PO4 is added to Ag+,Pb2+ and Sr2+ ions.

The selective precipitation is used to separate the cations from the solution.

Separation of mixtures of metal ions in aqueous solution using a reagent is known as selective precipitation.

Ca2+ will precipitate first, then Mg2+ and lastly Ba2+ .

The selective precipitation is used to separate the cations from the solution. The solubility product is inversely proportional to order of the precipitation. Higher the solubility product, lower will be the appearance of precipitate. The solubility product of some compounds is,

Refer Table 15.1 .

Ksp for BaF2 is 2.4×105 .

Ksp for MgF2 is 6.4×109 .

Ksp for CaF2 is 4.0×1011 .

From the above, Ksp values, it is clear that CaF2 has lowest Ksp value and BaF2 has maximum Ksp value. The order of precipitation of Mg2+,Ca2+ and Ba2+ is,

Ca2+FirstMg2+SecondBa2+Third

Pb2+ will precipitate first, then Sr2+ and lastly Ag+

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