Question

For each of the following salts, indicate if the solution would be acidic, basic or neutral. (a) \(\mathrm{NH}_{4} \mathrm{Cl}\) (b) \(\mathrm{KBr}\) (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\)

Short answer

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

Step-by-step solution

Analyze NH_4Cl

For \( \mathrm{NH}_{4} \mathrm{Cl} \), consider the ions it forms in solution: \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{Cl}^{-} \). \( \mathrm{NH}_{4}^{+} \) is a conjugate acid of a weak base (\( \mathrm{NH}_{3} \)), making the solution acidic. \( \mathrm{Cl}^{-} \) is a conjugate base of a strong acid (\( \mathrm{HCl} \)), and does not affect the pH. Thus, the solution will be acidic.

Analyze KBr

For \( \mathrm{KBr} \), it dissociates into \( \mathrm{K}^{+} \) and \( \mathrm{Br}^{-} \) ions. \( \mathrm{K}^{+} \) is the cation of a strong base (\( \mathrm{KOH} \)), and \( \mathrm{Br}^{-} \) is the anion of a strong acid (\( \mathrm{HBr} \)). Both ions do not affect the pH of the solution. Therefore, the solution is neutral.

Analyze Na_2CO_3

For \( \mathrm{Na}_{2} \mathrm{CO}_{3} \), it dissociates into \( \mathrm{Na}^{+} \) and \( \mathrm{CO}_{3}^{2-} \). \( \mathrm{Na}^{+} \) comes from a strong base (\( \mathrm{NaOH} \)) and does not affect the pH. \( \mathrm{CO}_{3}^{2-} \) is the conjugate base of a weak acid (\( \mathrm{H}_2\mathrm{CO}_3 \)), making the solution basic.

Analyze NaCH_3CO_2

For \( \mathrm{NaCH}_{3} \mathrm{CO}_{2} \), it dissociates into \( \mathrm{Na}^{+} \) and \( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \) ions. \( \mathrm{Na}^{+} \) does not affect the pH as it comes from \( \mathrm{NaOH} \), a strong base. \( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \) is the conjugate base of a weak acid (acetic acid, \( \mathrm{CH}_{3}\mathrm{COOH} \)), making the solution basic.

Key concepts

Salt Solutions

When salts dissolve in water, they break down into their constituent ions. These ions can influence the acidity or basicity of the solution. For instance, when \( \mathrm{NH}_{4} \mathrm{Cl} \) dissolves, it releases \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{Cl}^{-} \). The nature of these ions determines whether the solution is acidic, basic, or neutral.

In salt solutions:

• If the salt consists of the conjugate acid of a weak base, the solution tends to be acidic.
• If the salt has the conjugate base of a weak acid, the solution tends to be basic.
• If both ions come from strong acids and bases, the effect on pH is neutral.

This is why salts like \( \mathrm{KBr} \) result in neutral solutions, as both ions derive from strong counterparts.

pH Levels

pH is a scale used to determine the acidity or basicity of a solution, ranging from 0 to 14. A pH of 7 is neutral, under 7 is acidic, and above 7 is basic. Understanding the source of ions in a salt helps predict pH.

For example, the presence of \( \mathrm{NH}_{4}^{+} \) from \( \mathrm{NH}_{4} \mathrm{Cl} \) shifts the pH below 7, creating an acidic environment. Conversely, \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) with \( \mathrm{CO}_{3}^{2-} \) raises the pH above 7, making the solution basic.

Regular pH testing in chemistry helps determine the exact nature of a solution, affecting reactions and processes.

Conjugate Acid-Base Pairs

Every acid has a conjugate base and vice versa, forming a conjugate acid-base pair. This relationship is crucial in determining the behavior of salt solutions.

For instance, \( \mathrm{NH}_{4}^{+} \) is the conjugate acid of the weak base ammonium \( \mathrm{NH}_{3} \). It affects the solution by increasing its acidity when \( \mathrm{NH}_{4} \mathrm{Cl} \) dissolves. Meanwhile, \( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \) is the conjugate base of acetic acid, hence basic when \( \mathrm{NaCH}_{3} \mathrm{CO}_{2} \) dissolves.

Identifying these pairs helps predict whether a solution will be acidic or basic depending on the reaction materials involved.

Neutralization Reactions

Neutralization is a chemical reaction where an acid and a base react to form water and a salt. This process often affects the pH of the resulting solution.

The classic example involves reactions like when hydrochloric acid \( \mathrm{HCl} \) meets sodium hydroxide \( \mathrm{NaOH} \), producing water and sodium chloride \( \mathrm{NaCl} \), a neutral salt.

• Neutral salts like \( \mathrm{KBr} \) do not alter the pH significantly because they stem from strong acids and bases.
• Acidic or basic salts affect the pH based on their components, such as \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) from a weak acid, which becomes basic.

Understanding these reactions helps predict and control pH in various chemical and environmental systems.