Question

For which salt in each of the following groups will the solubility depend on pH? a. \(\mathrm{AgF}, \mathrm{AgCl}, \mathrm{AgBr}\) c. \(\mathrm{Sr}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{Sr}\left(\mathrm{NO}_{2}\right)_{2}\) b. \(\mathrm{Pb}(\mathrm{OH})_{2}, \mathrm{PbCl}_{2}\) d. \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{Ni}(\mathrm{CN})_{2}\)

Short answer

In each group, the salts with solubility depending on pH are: a. \(\mathrm{AgF}\) b. \(\mathrm{Pb}(\mathrm{OH})_{2}\) c. \(\mathrm{Sr}\left(\mathrm{NO}_{2}\right)_{2}\) d. \(\mathrm{Ni}(\mathrm{CN})_{2}\)

Step-by-step solution

Examine anions

We have three silver salts: silver fluoride, silver chloride, and silver bromide. Their anions are fluoride (F⁻), chloride (Cl⁻), and bromide (Br⁻), respectively. All these anions are conjugate bases of weak acids; thus, they are weak bases. However, among these three, the fluoride ion is the most basic.

Determine the effect on solubility

Since the anion fluoride (F⁻) is the most basic of the three, any changes in pH will have the most significant effect on the solubility of silver fluoride (AgF). Thus, the solubility of AgF depends on the pH. #b. \(\mathrm{Pb}(\mathrm{OH})_{2}, \mathrm{PbCl}_{2}\)

Examine anions

We have two lead salts: lead hydroxide and lead chloride. Their anions are hydroxide (OH⁻) and chloride (Cl⁻), respectively. Hydroxide is a strong base, and chloride is a weak base.

Determine the effect on solubility

Since the anion hydroxide (OH⁻) is a strong base, the solubility of lead hydroxide (Pb(OH)₂) will be affected by changes in pH. Thus, the solubility of Pb(OH)₂ depends on the pH. #c. \(\mathrm{Sr}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{Sr}\left(\mathrm{NO}_{2}\right)_{2}\)

Examine anions

We have two strontium salts: strontium nitrate and strontium nitrite. Their anions are nitrate (NO₃⁻) and nitrite (NO₂⁻), respectively. Nitrate is a conjugate base of a strong acid, so it is a weak conjugate base. Nitrite is a moderately weak base.

Determine the effect on solubility

Since the anion nitrite (NO₂⁻) is a moderately weak base, the solubility of strontium nitrite (Sr(NO₂)₂) will be affected by changes in pH. Thus, the solubility of Sr(NO₂)₂ depends on the pH. #d. \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{Ni}(\mathrm{CN})_{2}\)

Examine anions

We have two nickel salts: nickel nitrate and nickel cyanide. Their anions are nitrate (NO₃⁻) and cyanide (CN⁻), respectively. Nitrate is a conjugate base of a strong acid, so it is a weak conjugate base. Cyanide is a weak base.

Determine the effect on solubility

Since the anion cyanide (CN⁻) is a weak base, the solubility of nickel cyanide (Ni(CN)₂) will be affected by changes in pH. Thus, the solubility of Ni(CN)₂ depends on the pH.

Key concepts

Solubility Product

The solubility product constant, denoted as \(K_{sp}\), is a key concept in understanding salt solubility. It represents the maximum amount of a solid that can dissolve in a solution, reaching a state of dynamic equilibrium. For a salt AB that dissolves into A⁺ and B⁻ ions, its solubility product is defined as:\[ K_{sp} = [A^+][B^-] \]This equation shows that the solubility of a salt is directly tied to the product of the concentrations of its ions in a saturated solution. When soluble ionic compounds are introduced into a solution, they dissociate into their respective ions.

• High solubility means a large amount of salt can dissolve, having a high \(K_{sp}\).
• Low solubility results in a small \(K_{sp}\), indicating limited dissolving ability.

Factors like temperature and the presence of other ions can affect \(K_{sp}\). Understanding solubility product is critical, especially in cases where pH affects ion concentration.

Common Ion Effect

The common ion effect describes how the presence of a common ion in a solution can reduce the solubility of a salt. This happens because adding more of one of the ions shifts the equilibrium, following Le Chatelier's principle. For instance, if you have a saturated solution of silver chloride (AgCl), the addition of extra chloride ions (Cl⁻) from another source will decrease the solubility of AgCl. This is because the equilibrium shifts to favor the formation of more solid AgCl, reducing its concentration in solution.

• The common ion effect leads to a decrease in the dissolution of salts.
• It is commonly observed in solutions where salts have an ion in common.

This principle is important in controlling reactions, especially where manipulating ionic concentrations is necessary. It directly affects the solubility and is vital to consider when studying solutions with multiple salts.

Acid-Base Chemistry

Acid-base chemistry involves the study of hydrogen ion (H⁺) concentration changes in a solution and their corresponding effects on solubility. In the realm of pH-dependent solubility, we focus on how weak bases and their corresponding salts dissolved in water react with hydrogen ions. Changes in pH affect the dissociation state of compounds:

• Lower pH (acidic conditions) leads to more H⁺ ions, potentially increasing the solubility of salts with basic anions.
• Higher pH (basic conditions) reduces H⁺ availability, affecting salts containing acidic cations.

Understanding how acids and bases interact in solutions helps predict solubility changes based on pH fluctuations. This is crucial for more complex chemical processes and determining whether a salt will dissolve under certain conditions.

Weak Bases

Weak bases are species that do not completely ionize in solution. They are typically conjugate bases of weak acids and can influence the solubility of salts. Weak bases, such as fluoride (F⁻) or cyanide (CN⁻), produce a noticeable change in solubility depending on the pH of the environment. When a solution is acidic (low pH), weak bases react with available H⁺ ions, leading to increased solubility of their salts. For example, fluoride ions in silver fluoride (AgF) or cyanide ions in nickel cyanide (Ni(CN)₂) will be more soluble in acidic solutions:

• Weak base anions participate in equilibrium reactions, sensitive to [H⁺] concentration.
• They enhance solubility when more H⁺ is present, forming neutral molecules.

This interaction between weak bases and pH conditions is vital for predicting and controlling the behavior of salts in various chemical processes.