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AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumSolubility Equilibria

Solubility Equilibria (College Board AP® Chemistry): Exam Questions

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Solubility Equilibria

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1a1 mark

A student is studying the properties of CaSO4 and PbSO4 . The student has samples of both compounds, which are white powders.

The student tests the electrical conductivity of each solid and observes that neither solid conducts electricity. Describe the structures of the solids that account for their inability to conduct electricity.

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1b1 mark

The student places excess CaSO4 (s) in a beaker containing 100 mL of water and places excess PbSO4 (s) in another beaker containing 100 mL of water. The student stirs the contents of the beakers and then measures the electrical conductivity of the solution in each beaker. The student observes that the conductivity of the solution in the beaker containing the CaSO4 (s) is higher than the conductivity of the solution in the beaker containing the PbSO4 (s).

Which compound is more soluble in water, CaSO4 (s) or PbSO4 (s) ? Justify your answer based on the results of the conductivity test.

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1c1 mark

The left side of the diagram below shows a particulate representation of the contents of the beaker containing the CaSO4(s) from the solution conductivity experiment.

q6b-june-2021---ap-chemistry

Draw a particulate representation of PbSO4 (s) and the ions dissolved in the solution in the beaker on the right in the diagram. Draw the particles to look like those shown to the right of the beaker. Draw an appropriate number of dissolved ions relative to the number of dissolved ions in the beaker on the left.

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1d1 mark

The student attempts to increase the solubility of CaSO4 (s) by adding 10.0 mL of 2 M H2SO4 (aq) to the beaker, and observes that additional precipitate forms in the beaker. Explain this observation.

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2a1 mark

Barium sulfate (BaSO4​) is a sparingly soluble salt used in medical imaging. The solubility-product constant (Ksp​) for BaSO4​ at 25 oC is 1.1×10−10.

A student prepares a saturated solution of BaSO4 in water at 25 oC and wants to determine the molar solubility of the salt.

Write the dissolution equation and the Ksp​ expression for BaSO4​.

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2b2 marks

Calculate the molar solubility of BaSO4​ in pure water at 25 oC.

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2c1 mark

A saturated solution of BaSO4 is instead prepared in a 0.10 M solution of Na2SO4​. Predict whether the solubility of BaSO4​ will increase, decrease, or remain the same. Justify your answer based on equilibrium principles.

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3a1 mark

A student investigates whether a precipitate will form when mixing solutions of calcium chloride (CaCl2) and sodium sulfate (Na2SO4​). The solubility-product constant (Ksp​) for calcium sulfate (CaSO4​) at 25°C is 2.4 × 10−5.

The student mixes 50.0 mL of 0.0500 M CaCl2 and 50.0 mL of 0.0300 M Na2SO4​ in a beaker and observes the solution.

Write the dissolution equation and the Ksp​ expression for CaSO4​.

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3b1 mark

Determine the concentrations of Ca2+ and SO42−​ immediately after mixing.

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3c2 marks

i) Calculate the reaction quotient Q for CaSO4​ in the mixed solution.

ii) Compare Q to Ksp​ and predict whether a precipitate will form. Justify your answer.

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4a1 mark

The solubilty of three fluoride salts, LiF, CaF2 and MgF2, in water at 25 oC is investigated. The following data is collected:

Salt

Ksp (25 oC)

LiF

3.2 x 10-3

CaF2

3.9 x 10-11

MgF2

6.4 x 10-9

Write a balanced equation for the dissolution of CaF2 (s) in water.

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4b1 mark

State the mathematical expression that relates the solubility (S) of CaF2 to its Ksp.

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4c1 mark

Determine which of the three salts has the highest molar solubility.
Justify your answer.

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4d2 marks

An acidic solution is added to a saturated solution of CaF2. Explain how this affects the solubility of CaF2, considering that HF is a weak acid.

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4e2 marks

The dissolution of CaF2 is endothermic (ΔHosoln = +16.6 kJ mol-1).
Explain how increasing the temperature would affect the solubility of CaF2.

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4f3 marks

A saturated solution of CaF2 is mixed with a saturated solution of MgF2.

Predict whether a precipitate will form when the two solutions are combined. Justify your answer.

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1a1 mark

Answer the following questions about the solubility of Ca(OH)2  (Ksp = 1.3 × 10 −6).

Write a balanced chemical equation for the dissolution of Ca(OH)2 (s) in pure water.

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1b2 marks

Calculate the molar solubility of Ca(OH)2 in 0.10 M Ca(NO3)2 .

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1c1 mark

In the box below, complete a particle representation diagram that includes four water molecules with proper orientation around the Ca2+ ion.

Represent water molecules as

q4c2-june-2015---ap-chemistry
q4c-june-2015---ap-chemistry

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2a2 marks

Answer the following questions about Mg(OH)2. At 25°C, the value of the solubility product constant, Ksp for Mg(OH)2 (s) is  1.8 x 10−11 .

Calculate the number of grams of Mg(OH)2 (molar mass 58.32 g/mol) that is dissolved in 100 mL of a saturated solution of Mg(OH)2 at 25°C.

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2b2 marks

The energy required to separate the ions in the Mg(OH)2 crystal lattice into individual  Mg2+ (g) and OH −(g) ions, as represented in the table below, is known as the lattice energy of Mg(OH)2(s) . As shown in the table, the lattice energy of Sr(OH)2 (s) is less than the lattice energy of Mg(OH)2 (s) . Explain why in terms of periodic properties  and Coulomb's law.

Reaction

Lattice Energy (kJ/mol)

Mg(OH)2 (s) → Mg2+ (g) + 2OH− (g)

2900

Sr(OH)2 (s) → Sr2+ (g) + 2OH− (g)

2300

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3a1 mark
q7-june-2022-ap-chemistry

A Lewis electron-dot diagram of the oxalate ion, C2O42−, is shown.

Identify the hybridization of the valence orbitals of either carbon atom in the oxalate ion.

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3b3 marks

Silver oxalate, Ag2C2O4 (s), is slightly soluble in water. The value of Ksp for Ag2C2O4 is 5.40 × 10−12.

i) Write the expression for the solubility-product constant, Ksp, for Ag2C2O4.

ii) Calculate the molar solubility of Ag2C2O4 in neutral distilled water.

iii) The molar solubility of Ag2C2O4 increases when it is dissolved in 0.5 M  HClO4 (aq) instead of neutral distilled water. Write a balanced, net-ionic equation for the process that occurs between species in solution that contributes to the increased solubility of Ag2C2O4 (aq) in HClO4 (aq).

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4a1 mark

A student is investigating the solubility of silver chromate, Ag2CrO4, which forms a red precipitate in aqueous solution. The student prepares a saturated solution of Ag2CrO4 and uses it in a series of solubility tests at 25 oC.

Write a balanced equation, including state symbols, for the dissolution of Ag2CrO4 (s) in water at 25 oC.

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4b2 marks

The Ksp for Ag2CrO4 at 25 oC is 1.1 × 10-12.

Calculate the molar solubility of Ag2CrO4 in pure water at 25 oC.

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4c2 marks

A solution of 0.10 mol L-1 AgNO3 is added to a beaker containing the saturated Ag2CrO4 solution. Explain whether additional Ag2CrO4 will precipitate.

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4d2 marks

Another beaker contains a saturated solution of Ag2CrO4. Potassium chloride (KCl) is slowly added. Justify whether a precipitate will form.

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4e3 marks

A student claims that the solubility of Ag2CrO4 must be greater than that of BaSO4 because Ag+ and CrO42- are soluble ions according to the solubility rules.

Evaluate the validity of this claim using chemical reasoning.

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