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 compound that completely dissociates into its ions when dissolved in water, producing hydroxide ions (OH⁻) and increasing the pH of the solution. (b) For the \(0.035 M\) Sr(OH)\(_2\) solution, \([OH^{-}] = 2 × 0.035 M = 0.07 M\). (c) The statement is false. Mg(OH)\(_2\) can still dissociate completely into Mg\(^{2+}\) and OH⁻ ions when dissolved, making it a strong base despite its low solubility.

Step-by-step solution

(a) Definition of strong base

A strong base is a compound that completely dissociates into its ions when dissolved in water, producing hydroxide ions (OH⁻) and increasing the pH of the solution.

(b) Calculate the [OH⁻]

To calculate the concentration of hydroxide ions \([\mathrm{OH}^{-}]\) for the \(0.035 M\) Sr(OH)\(_2\) solution, we need to consider the stoichiometry of the dissociation reaction. In the dissociation of Sr(OH)\(_2\), it produces two hydroxide ions for each molecule: \[ \mathrm{Sr(OH)_{2}} \rightarrow \mathrm{Sr^{2+}} + 2\mathrm{OH^-} \] Since 1 molecule of Sr(OH)\(_2\) produces 2 molecules of OH⁻, the \([OH^-]\) in the solution will be twice the concentration of Sr(OH)\(_2\): \([OH^{-}] = 2 × 0.035 M = 0.07 M\)

(c) Statement verification and explanation

The statement is false. A strong base is defined by how much it can dissociate into its ions in water, not by its solubility. While Mg(OH)\(_2\) is not very soluble, it can still dissociate completely into Mg\(^{2+}\) and OH⁻ ions when it does dissolve. Therefore, Mg(OH)\(_2\) can be considered a strong base.

Key concepts

Dissociation

Dissociation is a process in chemistry where a compound breaks down into its component ions when dissolved in water. This is a key property of strong bases, as they completely dissociate. The dissociation process allows the base to release hydroxide ions
(\(\mathrm{OH^-}\)) into the solution. For instance, when \(\mathrm{Sr(OH)_{2}}\) is dissolved in water, it dissociates completely:

• Every molecule of \(\mathrm{Sr(OH)_{2}}\) splits into one \(\mathrm{Sr^{2+}}\) ion and two \(\mathrm{OH^-}\) ions.

This total dissociation is what characterizes strong bases like \(\mathrm{Sr(OH)_{2}}\), as it fully breaks down in water, increasing the concentration of hydroxide ions in the solution.

Hydroxide Ions

Hydroxide ions (\(\mathrm{OH^-}\)) are negatively charged ions that play a crucial role in defining the strength of a base. They are produced during the dissociation of bases in water. Strong bases release a high concentration of hydroxide ions, which is directly correlated to their ability to increase the pH of the solution. Every strong base, like \(\mathrm{Sr(OH)_{2}}\), adds these ions to the solution. For \(\mathrm{Sr(OH)_{2}}\), it releases twice as many hydroxide ions as its own concentration, meaning a \(0.035 \, \mathrm{M}\) solution of \(\mathrm{Sr(OH)_{2}}\) results in a \(0.07 \, \mathrm{M}\) concentration of \(\mathrm{OH^-}\) ions.

Solubility

Solubility refers to how well a substance can dissolve in a solvent, such as water. It's important to note that solubility and the strength of a base are not the same thing. For example, some compounds like \(\mathrm{Mg(OH)_{2}}\) have low solubility in water, meaning it doesn't dissolve well. However, this doesn't imply it's a weak base. Once dissolved, \(\mathrm{Mg(OH)_{2}}\) fully dissociates into its ions:

• It releases \(\mathrm{Mg^{2+}}\) and \(\mathrm{OH^-}\) ions.

So, despite its low solubility, it behaves like a strong base. This means people sometimes confuse a compound's solubility with its ability to fully dissociate in water.

pH

The pH scale ranges from 0 to 14 and measures the acidity or basicity of a solution. A solution with a low pH is acidic, while a solution with a high pH is basic. Hydroxide ions (\(\mathrm{OH^-}\)) released by strong bases increase the pH, shifting the balance towards basicity. When you add a strong base to a solution and it produces a high concentration of \(\mathrm{OH^-}\), it increases the pH significantly. For instance, if a liter of water with pH 7 has \(\mathrm{Sr(OH)_{2}}\) added, it will dissociate and increase the concentration of \(\mathrm{OH^-}\) ions, raising the solution's pH and making it basic.

Stoichiometry

Stoichiometry in chemistry involves calculating the quantities of reactants and products involved in a reaction. In the case of dissociation of strong bases, stoichiometry helps determine how many hydroxide ions are produced from each formula unit. With \(\mathrm{Sr(OH)_{2}}\), stoichiometry shows that each molecule releases two hydroxide ions. Therefore, if you have a solution of \(0.035 \, \mathrm{M}\) \(\mathrm{Sr(OH)_{2}}\), it results in \(0.07 \, \mathrm{M}\) of \(\mathrm{OH^-}\) ions as it fully dissociates. This ratio, dictated by stoichiometry, helps us in understanding the concentrations in chemical reactions involving bases.