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: Fluoridation of drinking water is employed in many places to aid in the prevention of dental caries. Typically the F- ion concentration is adjusted to about 1 ppb. Some water supplies are also "hard";

Problem

Fluoridation of drinking water is employed in many places to aid in the prevention of dental caries. Typically the F- ion concentration is adjusted to about 1 ppb. Some water supplies are also "hard"; that is, they contain certain cations such as Ca2+ that interfere with the action of soap. Consider a case where the concentration of Ca2+ is 8 ppb.

Could a precipitate of CaF2 form under these conditions? (Make any necessary approximations.)

Solution

In this problem, we are asked if CaF2 precipitate would form under the conditions set in the problem above.

For this, we need to compare the reaction quotient (Q) vs. the solubility product constant (Ksp)

Recall that when:

 Q > Ksp: the solution is supersaturated and a precipitate will form. Reactants are favored.

Q = Ksp: the solution is at equilibrium and no precipitate will form.

Q < Ksp: the solution is unsaturated and no precipitate will form. Products are favored.

To solve this problem:

Step 1. Convert the concentrations from ppb to M.

Step 2. Write and balance the disassociation of the compound into ions.

Step 3. Solve for Q using I.C.E. chart.

Step 4. Compare the Q value with the Ksp.

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