Problem 42 - Chemistry: The Central Science

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 completely dissociate in water, producing hydroxide ions (OH-). Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH). (b) True: The addition of a strong base to water produces a solution with a pH > 7.0, because the concentration of hydroxide ions (OH-) becomes greater than the concentration of hydrogen ions (H+), making the solution more basic. (c) False: Magnesium hydroxide (Mg(OH)2) is not a strong base, as it is sparingly soluble and does not dissociate completely in water. Being not very soluble does not necessarily mean it cannot be a strong base, but in this case, it is a weak base.

Step 1: Determine if all strong bases are salts of the hydroxide ion.

Strong bases are bases that completely dissociate in water, producing hydroxide ions (OH-). Examples of strong bases include alkali metal hydroxides, like sodium hydroxide (NaOH) and potassium hydroxide (KOH), and some alkaline earth metal hydroxides, like calcium hydroxide (Ca(OH)2). These strong bases are indeed salts of the hydroxide ion. In our case, all strong bases that we've mentioned are salts of hydroxide ions. Therefore, the statement (a) is true.

Step 2: Determine if the addition of a strong base to water produces a solution of pH > 7.0.

A solution is considered basic if the concentration of hydroxide ions (OH-) is greater than the concentration of hydrogen ions (H+). In such a case, the pH of the solution will be greater than 7. When a strong base, which completely dissociates in water, is added to water, it increases the concentration of hydroxide ions in the solution, making it more basic. As a result, the pH of the solution will be greater than 7.0. Therefore, the statement (b) is true.

Step 3: Determine if Mg(OH)2 being not very soluble means it cannot be a strong base.

Magnesium hydroxide, Mg(OH)2, is a sparingly soluble base, meaning that it does not dissociate completely in water. It forms a small amount of hydroxide ions when dissolved in water, making it a weak base, not a strong one. A strong base must be one that completely dissociates in a solution. Therefore, the statement (c) is false.

Key Concepts

Hydroxide Ions

Hydroxide ions, often represented as OH⁻, play a pivotal role in determining the basicity of a solution. When a substance known as a base dissociates in water, one of the products of this process is the hydroxide ion. This process increases the basic nature of the solution. For example, strong bases such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) will completely dissociate and produce a large amount of hydroxide ions.

These ions are essential for the characteristics of strong bases. They ensure that any solution they are added to becomes more basic, due to the increased presence of OH⁻. It's worth noting that not all substances containing hydroxide are strong bases. The ability of the substance to dissociate completely in water is what determines the strength of a base.

pH Levels

Understanding pH levels is crucial when studying the nature of acids and bases. The pH scale ranges from 0 to 14 and is used to quantify the acidity or basicity of an aqueous solution.

On this scale:

• A pH less than 7 indicates an acidic solution.
• A pH of exactly 7 indicates a neutral solution, like pure water.
• A pH greater than 7 indicates a basic (or alkaline) solution.

When strong bases are added to water, they increase the concentration of hydroxide ions, which in turn increases the pH level. As a result, any solution to which a strong base is added will have a pH level above 7, making it basic. This relationship between hydroxide ions and pH is fundamental to understanding how basic solutions behave in chemistry.

Solubility of Compounds

The solubility of a compound in water determines how well it can dissolve and subsequently dissociate to form ions. In the context of bases, solubility greatly influences whether a base is classified as strong or weak.

Strong bases, like NaOH and KOH, are highly soluble; they completely dissolve in water, producing a significant amount of hydroxide ions. In contrast, some compounds, like magnesium hydroxide (Mg(OH)₂), are not very soluble. This limited solubility results in only a small fraction of the compound dissociating in water. As such, Mg(OH)₂ is considered a weak base despite containing hydroxide ions, because it does not significantly increase the pH of the solution.

When considering whether a base is strong or weak, one must look at both solubility and the degree of dissociation. Only those bases that both dissolve and dissociate fully in water are classified as strong bases.