Ch.16 - Aqueous Equilibrium WorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Solution: Part A. A solution of Na 2CO3 is added dropwise to a solution that contains 1.14 x 10 -2 M Fe2+ and 1.50 x 10-2 M Cd 2+. What concentration of CO32- is needed to initiate precipitation? Neglect any volume changes during the addition. Express the molar concentration numerically. Part B. In the solution from Part A, what will the concentration of CO 32- be when Fe2+ begins to precipitate? Express the molar concentration numerically. Part C. What will the concentration of Cd 2+ be when Fe2+ begins to precipitate?

Problem

Part A. A solution of Na 2CO3 is added dropwise to a solution that contains 1.14 x 10 -2 M Fe2+ and 1.50 x 10-2 M Cd 2+. What concentration of CO32- is needed to initiate precipitation? Neglect any volume changes during the addition. Express the molar concentration numerically. 

Part B. In the solution from Part A, what will the concentration of CO 32- be when Fe2+ begins to precipitate? Express the molar concentration numerically. 

Part C. What will the concentration of Cd 2+ be when Fe2+ begins to precipitate?

Solution

A) For this part, we’re being asked to calculate the concentration of CO32– needed in order to precipitate initiate precipitation.


2 compounds are able to precipitate in this solution. CO32- combines with either Fe2+ or Cd 2+. Now let us look at dissociation reactions for both. 


FeCO3(s)Fe2+(aq) + CO32- (aq)


CdCO3(s)Cd2+(aq) + CO32- (aq)


Now we can see here that both reactions only form 2 ions when it dissociates. 

This means that we can directly compare the Ksp values to predict solubility. 

The lower the Ksp, the less soluble and the least soluble among the two will precipitate out first. 

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