Question

Classify each of the following salt solutions as acidic, basic, or neutral: (a) \(\mathrm{KBr}\) (b) \(\mathrm{NaNO}_{2}\) (c) \(\mathrm{NH}_{4} \mathrm{Br}\) (d) \(\mathrm{ZnCl}_{2}\) (e) \(\mathrm{NH}_{4} \mathrm{~F}\)

Short answer

(a) Neutral, (b) Basic, (c) Acidic, (d) Acidic, (e) Acidic

Step-by-step solution

Analyzing Salt Components

To determine if a salt solution is acidic, basic, or neutral, we examine the ions produced when the salt dissolves in water. Identify whether the cation comes from a strong or weak base and if the anion comes from a strong or weak acid.

Classifying Each Salt

(a) **KBr**: Potassium (\(\mathrm{K}^+\)) comes from the strong base \(\mathrm{KOH}\), and bromide (\(\mathrm{Br}^-\)) from the strong acid \(\mathrm{HBr}\). The solution is neutral. (b) **NaNO₂**: Sodium (\(\mathrm{Na}^+\)) comes from a strong base \(\mathrm{NaOH}\), and nitrite (\(\mathrm{NO}_2^-\)) from the weak acid \(\mathrm{HNO_2}\). The solution is basic. (c) **NH₄Br**: Ammonium (\(\mathrm{NH_4}^+\)) comes from the weak base \(\mathrm{NH_3}\), and bromide (\(\mathrm{Br}^-\)) from a strong acid \(\mathrm{HBr}\). The solution is acidic. (d) **ZnCl₂**: Zinc atoms can form acidic solutions when hydrated, and chloride (\(\mathrm{Cl}^-\)) comes from the strong acid \(\mathrm{HCl}\). The solution is acidic.(e) **NH₄F**: Ammonium (\(\mathrm{NH_4}^+\)) stems from a weak base \(\mathrm{NH_3}\), and fluoride (\(\mathrm{F}^-\)) from a weak acid \(\mathrm{HF}\). Comparing their \(pK_a\) values indicates the solution will be acidic, as ammonium is a stronger acid than fluoride is a base.

Summary of Results

Evaluate each salt based on the analyses: - **KBr**: Neutral - **NaNO₂**: Basic - **NH₄Br**: Acidic - **ZnCl₂**: Acidic - **NH₄F**: Acidic

Key concepts

Salt Solutions

Salt solutions are formed when salts dissolve in water, producing ions. The nature of these ions determines whether the solution is acidic, basic, or neutral. This process involves identifying the origin of the cation and anion.

• Cations usually originate from bases. If the base is strong, like sodium hydroxide (\(\mathrm{NaOH}\)), the cation \(\mathrm{Na}^+\) will not affect the pH.
• Anions typically come from acids. An anion from a strong acid, like chloride \(\mathrm{Cl}^-\) from hydrochloric acid (\(\mathrm{HCl}\)), will not change the pH either.

The interaction between these ions and water establishes whether the solution becomes acidic, basic, or neutral. Let's explore these classifications further.

Acidic Solutions

Acidic solutions occur when the solution has more hydrogen ions (\(\mathrm{H}^+\)). In salt solutions, this results from ions that tend to donate protons (acids) more readily.

• The presence of a weak base contributing cations like ammonium (\(\mathrm{NH_4}^+\)) can make a solution acidic. Ammonium, when dissolved, can release \(\mathrm{H}^+\), lowering the pH of the solution.
• An example is \(\mathrm{NH_4Br}\), with \(\mathrm{NH_4}^+\) from a weak base and \(\mathrm{Br}^-\) from a strong acid.
• When both the cation and anion are weak acid/base pairs, such as in \(\mathrm{NH_4F}\), the pH depends on the relative strengths. Here, the solution is acidic as \(\mathrm{NH_4}^+\) is a stronger acid than \(\mathrm{F}^-\) is a base.

Understanding these interactions helps to predict the acidity of the solution.

Basic Solutions

A basic solution has a higher concentration of hydroxide ions (\(\mathrm{OH}^-\)). This occurs when the anion from a weak acid can accept protons, increasing the \(\mathrm{OH}^-\) concentration.

• Sodium nitrite (\(\mathrm{NaNO_2}\)) is a perfect example where the nitrite ion (\(\mathrm{NO_2}^-\)) comes from a weak acid (\(\mathrm{HNO_2}\)). This ion tends to capture protons from water, leading to an increased \(\mathrm{OH}^-\) concentration and a basic solution.
• This phenomenon occurs because \(\mathrm{Na^+}\), coming from a strong base, doesn't react with water, allowing \(\mathrm{NO_2}^-\) to dominate and make the solution basic.

By recognizing these patterns, predicting when a salt solution will be basic becomes intuitive.

Neutral Solutions

Neutral solutions are those in which the solution's pH remains unchanged by the dissolved salt. This results when both ions in the salt come from equally strong acids and bases.

• Potassium bromide (\(\mathrm{KBr}\)) is one such salt where \(\mathrm{K}^+\) derives from a strong base (\(\mathrm{KOH}\)) and \(\mathrm{Br}^-\) from a strong acid (\(\mathrm{HBr}\)).
• Because neither ion interacts with water to change the concentration of \(\mathrm{H}^+\) or \(\mathrm{OH}^-\), the solution remains neutral with a pH around 7.

Recognizing these salt solutions is important in chemistry, as they provide a stable medium that doesn’t alter pH.