Problem 42 - Chemistry: The Central Science in SI Units, Global Edition

Question

Determine whether each of the following is true or false: (a) All strong bases are salts of the hydroxide ion. (b) The addition of a strong base to water produces a solution of \(\mathrm{pH}>7.0\) (c) Because \(\mathrm{Mg}(\mathrm{OH})_{2}\) is not very soluble, it cannot be a strong base.

Step-by-Step Solution

Answer

(a) True: All strong bases are salts of the hydroxide ion, as they fully ionize to produce OH- ions in solution. (b) True: The addition of a strong base to water produces a solution of pH > 7.0 due to the increased concentration of OH- ions. (c) False: Despite its low solubility, Mg(OH)2 can still be a strong base since it fully ionizes into Mg2+ and OH- ions when dissolved in water.

Step 1: Statement (a): All strong bases are salts of the hydroxide ion.

A strong base is a chemical compound that will fully ionize when dissolved in water, increasing the concentration of hydroxide ions (OH-) in the solution. Salts of the hydroxide ion are formed when a metal ion combines with OH-. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are common strong bases. Since strong bases fully ionize to produce hydroxide ions, it can be considered true that all strong bases are salts of the hydroxide ion.

Step 2: Statement (b): The addition of a strong base to water produces a solution of pH > 7.0.

The pH scale ranges from 0 to 14 and measures how acidic or basic a solution is. A pH of 7 is neutral, while values below 7 indicate an acidic solution and values above 7 indicate a basic (alkaline) solution. When a strong base is added to water, it ionizes fully to produce a high concentration of OH- ions. This increased concentration of OH- ions causes a rise in pH, resulting in a pH greater than 7, which confirms that this statement is true.

Step 3: Statement (c): Because Mg(OH)2 is not very soluble, it cannot be a strong base.

A strong base refers to the ability of a compound to fully ionize when dissolved in water, increasing the concentration of hydroxide ions (OH-) in the solution. Magnesium hydroxide (Mg(OH)2) has low solubility in water, which means that only a small amount of the compound will dissolve in water. However, when Mg(OH)2 dissolves, it ionizes fully and forms Mg2+ and OH- ions. Although the solubility is low, the fact that it fully ionizes upon dissolving indicates that it can still be considered as a strong base. Therefore, this statement is false.

Key Concepts

Hydroxide Ion

A hydroxide ion is a negatively charged molecule consisting of one oxygen atom bonded to one hydrogen atom. Its chemical formula is OH-. This ion is critical in the chemistry of bases. When a substance is a strong base, it dissociates completely in water to release hydroxide ions.

For example, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are strong bases that dissolve in water, yielding a high number of hydroxide ions, thus increasing the basicity of the solution.

Some key points about hydroxide ions include:

• They make solutions basic when present in high concentrations.
• They react readily with protons (H+) to form water (H2O).
• Their presence often characterizes strong bases.

Understanding the behavior of hydroxide ions helps explain why bases, especially strong ones, increase pH in aqueous solutions.

pH Scale

The pH scale ranges from 0 to 14 and is a measure of how acidic or basic a solution is. The scale is logarithmic, meaning each whole number change represents a tenfold increase or decrease in hydrogen ion concentration.

Here's how the scale is generally interpreted:

• A pH of 7 is neutral — like pure water.
• A pH less than 7 is considered acidic, with lower numbers indicating stronger acidity.
• A pH greater than 7 is basic (alkaline), with higher numbers suggesting stronger basicity.

When a strong base is introduced into water, it dissociates fully, producing more hydroxide ions. These ions lower the concentration of hydrogen ions, increasing the pH and making the solution more basic (pH > 7). Understanding the pH scale is essential for working with solutions in chemistry.

Solubility

Solubility describes the ability of a substance to dissolve in a solvent. In our context, it refers to how well a base like magnesium hydroxide (Mg(OH)2) can dissolve in water.

Though Mg(OH)2 is not very soluble in water, it is still classified as a strong base. This is because, despite low solubility, whatever amount of Mg(OH)2 does dissolve will ionize completely.

Key aspects of solubility include:

• The amount of solute that can dissolve in a certain volume of solvent at a specified temperature.
• Factors affecting solubility, such as temperature and the nature of both solute and solvent.
• Strong bases that have low solubility can still fully ionize in the dissolved portion, contributing to the concentration of hydroxide ions.

Understanding solubility helps clarify why a substance with low solubility can still be considered a strong base.