Question

(a) What is a strong base? (b) A solution is labeled \(0.035 \mathrm{M}\) \(\mathrm{Sr}(\mathrm{OH})_{2} .\) What is \(\left[\mathrm{OH}^{-}\right]\) for the solution? (c) Is the following statement true or false? Because \(\mathrm{Mg}(\mathrm{OH})_{2}\) is not very soluble, it cannot be a strong base. Explain.

Short answer

(a) A strong base is a substance that completely dissociates in an aqueous solution to form a high concentration of hydroxide ions (OH⁻) and the resulting conjugate acid. (b) The concentration of hydroxide ions in the given \(Sr(OH)_2\) solution is 0.07 M. (c) The statement is false. Mg(OH)₂ is not very soluble, but once it dissolves, it dissociates completely, making it a strong base. Its low solubility affects the amount of OH⁻ ions it can provide in a given solution, but this is not directly related to its strength as a base.

Step-by-step solution

(a) Definition of a Strong Base

A strong base is a substance that readily donates hydroxide ions (OH⁻) when dissolved in water to form a highly alkaline solution. It dissociates completely in an aqueous solution, which means that the reaction between the strong base and water goes to completion, producing a high concentration of hydroxide ions and the resulting conjugate acid.

(b) Calculate [OH⁻] for the given solution

In order to calculate the concentration of hydroxide ions, we need to take into account the stoichiometry of strontium hydroxide's dissolution in water. The equation for this dissociation is: \[Sr(OH)_2 \longleftrightarrow Sr^{2+} + 2OH^-\] As we see, one mole of \(Sr(OH)_2\) dissociates to produce two moles of hydroxide ions. Hence, to find the concentration of hydroxide ions, we should multiply the molarity of strontium hydroxide by 2. Given, the molarity of the \(Sr(OH)_2\) solution is 0.035 M. Therefore, the concentration of hydroxide ions is: \[[OH^-] = 2 \times 0.035\] \[[OH^-] = 0.07\,M\]

(c) Evaluation of the statement

The statement is false. The solubility and the strength of a base are two different properties. Solubility refers to the amount of a substance that can dissolve in a given solvent, whereas the strength of a base refers to how much it can increase the hydroxide ion concentration in the solution. Mg(OH)₂ is not very soluble, but once it dissolves in water, it dissociates completely. It produces hydroxide ions (OH⁻) according to the equation: \[Mg(OH)_2 \longleftrightarrow Mg^{2+} + 2OH^-\] The fact that Mg(OH)₂ is not very soluble does not mean it cannot be a strong base. Once it dissolves and dissociates, it can still produce a high concentration of hydroxide ions, making it a strong base. Its low solubility mainly affects the amount of OH⁻ ions it can provide in a given solution, but this is not directly related to its strength as a base.

Key concepts

Hydroxide Ions

Hydroxide ions, denoted as \(OH^-\), play a crucial role in determining the alkalinity of a solution. These ions are characterized by their negative charge, consisting of one oxygen and one hydrogen atom. They are pivotal in the action of bases, as they help to neutralize acids by reacting with hydrogen ions \((H^+)\) to form water.

When a substance that is classified as a base dissolves in water, it tends to release hydroxide ions. The degree to which these ions are released is what dictates the strength of the base. Strong bases are particularly effective because they completely dissociate to maximize the concentration of \(OH^-\) in a solution.

Thus, understanding the behavior of hydroxide ions in solution is essential for grasping the nature of alkaline environments and the efficacy of bases.

Solubility and Base Strength

The concepts of solubility and base strength, while related, refer to distinct properties that should not be confused. Solubility measures how well a substance dissolves in a solvent, typically water. A highly soluble compound will dissolve more readily to achieve a higher concentration of ions in the solution.

On the other hand, the strength of a base is its ability to generate hydroxide ions when dissolved. A strong base will dissociate completely in water, leading to a high concentration of \(OH^-\) ions, regardless of how much solute dissolves.

Some bases, like \(Mg(OH)_2\), have low solubility but high dissociation capacity. When such a compound does dissolve, it fully releases its available hydroxide ions into the solution, behaving as a strong base. Therefore, the seemingly low solubility of a compound doesn't always equate to diminished strength as a base.

Aqueous Solution Dissociation

Dissociation in an aqueous solution is a process integral to understanding how bases behave when mixed with water. When a compound dissolves in water, it breaks down into its constituent ions. This ionic separation is what dissociation entails.

For strong bases, this process is complete. Consider a base like \(Sr(OH)_2\) which, when dissolved in water, dissociates entirely into \(Sr^{2+}\) ions and \(OH^-\) ions. The stoichiometry of the dissociation process shows that each formula unit of a strong base provides a substantial number of hydroxide ions.

This thorough dissociation is why strong bases effectively increase the \(OH^-\) concentration in a solution. Through achieving full ionic separation, strong bases exhibit maximal alkaline behavior, which is a hallmark of their strength as bases.