Question

Explain with equations and calculations, when necessary, whether an aqueous solution of each of these salts is acidic, basic, or neutral: (a) \(\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Cl} ;\) (b) \(\mathrm{LiClO}_{4} ;\) (c) \(\mathrm{CoF}_{2}\).

Short answer

The solutions are: (a) acidic, (b) neutral, (c) slightly acidic.

Step-by-step solution

Analyze \(\mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl}\)

Consider \(\mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl}\) which is composed of \(\mathrm{CH}_3\mathrm{NH}_3^+\) (methylammonium ion) and \(\mathrm{Cl}^-\) (chloride ion). \(\mathrm{CH}_3\mathrm{NH}_3^+\) is a conjugate acid of a weak base (methylamine), and \(\mathrm{Cl}^-\) is the conjugate base of a strong acid (hydrochloric acid), contributing no basic properties.

Determine the Effect on pH for \(\mathrm{CH}_3\mathrm{NH}_3\mathrm{Cl}\)

Since \(\mathrm{CH}_3\mathrm{NH}_3^+\) is the conjugate acid of a weak base, it tends to donate \(\mathrm{H}^+\) ions in the solution, making the solution acidic. \(\mathrm{Cl}^-\) does not affect the pH significantly.

Analyze \(\text{LiClO}_4\)

Consider \(\mathrm{LiClO}_{4}\) which is composed of \(\mathrm{Li}^+\) (lithium ion) and \(\mathrm{ClO}_4^-\) (perchlorate ion). \(\mathrm{Li}^+\) comes from a strong base (lithium hydroxide), and \(\mathrm{ClO}_4^-\) comes from a strong acid (perchloric acid). Neither ion hydrolyzes significantly in water.

Determine the Effect on pH for \(\mathrm{LiClO}_{4}\)

Since both \(\mathrm{Li}^+\) and \(\mathrm{ClO}_4^-\) are from strong acid and base respectively, the solution remains neutral.

Analyze \(\text{CoF}_2\)

Consider \(\mathrm{CoF}_{2}\) which is composed of \(\mathrm{Co}^{2+}\) (cobalt ion) and \(\mathrm{F}^-\) (fluoride ion). \(\mathrm{Co}^{2+}\) is a metal ion with a relatively high charge density, leading to hydrolysis and release of \(\mathrm{H}^+\) ions. \(\mathrm{F}^-\) is a conjugate base of a weak acid (hydrofluoric acid), slightly basic.

Determine the Effect on pH for \(\mathrm{CoF}_2\)

Since \(\mathrm{Co}^{2+}\) causes hydrolysis and increases \(\mathrm{H}^+\) concentration, and \(\mathrm{F}^-\) slightly counteracts this by being weakly basic, the solution's overall effect tends to be slightly acidic.

Key concepts

aqueous solutions

An aqueous solution is simply a solution where the solvent is water. Chemical compounds in these solutions can dissociate into ions. These ions can affect the properties of the solution, including its acidity or basicity.
When salts dissolve in water, they dissociate into their constituent ions. The behavior of these ions in water depends on their nature and their origin. For instance, ions coming from strong acids or bases do not significantly affect the pH, while those from weak acids or bases do.
A typical example is sodium chloride (table salt), which dissociates into \(\text{Na}^+\) and \(\text{Cl}^-\) ions. Since both come from a strong acid (hydrochloric acid) and a strong base (sodium hydroxide), they do not change the pH markedly.
Understanding the behavior of these ions in water underpins many concepts in chemistry, including how to identify if a solution is acidic, basic, or neutral.

pH determination

The pH of a solution is a measure of its acidity or basicity. It ranges from 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is basic. The pH is calculated using the concentration of hydrogen ions \(\text{H}^+\) in the solution: \(\text{pH} = -\log[\text{H}^+]\).
In the analysis of whether a salt solution is acidic, basic, or neutral, the pH provides a clear indicator. For example, a lower pH reflects a higher concentration of \(\text{H}^+\) ions—a characteristic of an acidic solution.
To determine the pH, consider the ions each salt dissociates into and their propensities to either release \(\text{H}^+\) or \(\text{OH}^-\). This involves understanding the properties of conjugate acids and bases and how they interact with water. Thus, determining pH is crucial for categorizing a solution correctly.

conjugate acids and bases

Conjugate acids and bases are pairs of compounds that transform into each other by gaining or losing a proton (\(\text{H}^+\)). For instance, when a base gains a proton, it forms its conjugate acid, and when an acid loses a proton, it forms its conjugate base.
For example, \(\text{CH}_3\text{NH}_3^+\) is the conjugate acid of \(\text{CH}_3\text{NH}_2\) (a weak base), and \(\text{Cl}^-\) is the conjugate base of hydrochloric acid (a strong acid).
This concept explains why \(\text{CH}_3\text{NH}_3\text{Cl}\) forms an acidic solution. \(\text{CH}_3\text{NH}_3^+\) tends to donate protons to the solution, increasing the concentration of \(\text{H}^+\) ions and thus lowering the pH.
Understanding the nature of conjugate acids and bases helps predict whether the dissolution of a particular salt will result in an acidic, basic, or neutral solution.

hydrolysis

Hydrolysis refers to a reaction involving water, where a compound reacts with water, leading to the dissociation of water molecules. In the context of salts, hydrolysis can affect the pH of the solution significantly.
Consider \(\text{CoF}_2\). Here, the \(\text{Co}^{2+}\) ion hydrolyzes, reacting with water to produce \(\text{H}^+\) ions, making the solution more acidic. This process increases the free hydrogen ion concentration in the solution.
Conversely, the \(\text{F}^-\) ion slightly counteracts this effect because it is the conjugate base of a weak acid (hydrofluoric acid), and it tends to removing some \(\text{H}^+\) ions. However, this equilibrium tilts more towards acidity when \(\text{CoF}_2\) is dissolved.
Recognizing how different ions undergo hydrolysis helps in predicting the resultant pH of a solution, linking to larger concepts of acidity and basicity in aqueous solutions.