Question 1

What is the common ion effect and how does it reduce solubility?

Accepted Answer

The common ion effect occurs when a solute ion already present in solution shifts the solubility equilibrium of a sparingly soluble salt. When sodium chloride is added to a lead(II) chloride solution, the increased chloride concentration causes the equilibrium to shift left, forcing more lead(II) chloride to remain undissolved. This reduces solubility because the ion product must remain constant at equilibrium.

Question 2

How do you calculate molar solubility using an ICE table and Ksp?

Accepted Answer

Set up an ICE table tracking initial concentrations, changes, and equilibrium values for each ion. For lead(II) chloride in sodium chloride solution, the chloride initial concentration is 0.100 M, and the change is +2x per formula unit. Substitute equilibrium concentrations into the Ksp expression and solve for x, the molar solubility.

Question 3

How does pH affect the solubility of compounds containing hydroxide ions?

Accepted Answer

Adding acid decreases pH by consuming hydroxide ions, shifting the calcium hydroxide equilibrium toward dissolution and increasing solubility. Conversely, adding base increases pH and hydroxide concentration, shifting equilibrium left and decreasing solubility. The solubility of hydroxide-containing compounds is inversely related to solution pH.

Question 4

Why is selective precipitation used in wastewater treatment?

Accepted Answer

Selective precipitation removes specific contaminants like phosphate ions by adding reagents such as calcium hydroxide. The calcium ions react with phosphates to form hydroxylapatite, which precipitates out of solution. This method selectively removes target ions while leaving other dissolved salts behind, allowing filtration to separate the solid precipitate.

Question 5

What is the quantitative relationship between ion concentrations at equilibrium?

Accepted Answer

The solubility product constant (Ksp) expresses the mathematical product of ion molarities at equilibrium. For lead(II) chloride, Ksp equals [Pb2+][Cl−]2. This product remains constant regardless of ion source, so increasing one ion concentration must proportionally decrease the other to maintain equilibrium.

Question 6

How much does the common ion effect reduce lead(II) chloride solubility in sodium chloride solution?

Accepted Answer

In 0.100 M sodium chloride solution, lead(II) chloride solubility decreases to 1.17 × 10−3 M compared to 1.43 × 10−2 M in pure water, a 12-fold reduction. This dramatic decrease demonstrates how the common chloride ion shifts the dissolution equilibrium significantly toward the undissociated salt.

Question 7

What happens to solubility when multiple calcium salts compete for calcium ions?

Accepted Answer

When calcium hydroxide is added to wastewater containing phosphates, calcium ions preferentially precipitate phosphates as hydroxylapatite because this reaction is favored. The amount of calcium added is controlled so other calcium salt solubility products are not exceeded, allowing their anions to remain dissolved and be separated later.

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