Question

The aqueous solution of a salt is alkalinc. This shows that salt is made from (1) \(\Lambda\) strong acid and strong base (2) \(\Lambda\) strong acid and weak base (3) \(\Lambda\) wcak acid and weak base (4) \(\Lambda\) weak acid and strong base

Short answer

Option (4) \Lambda\ weak acid and strong base

Step-by-step solution

Understand the Problem

We need to determine the combination of acid and base that results in an alkaline solution when dissolved in water.

Define an Alkaline Solution

An alkaline solution has a pH greater than 7. This means it has more hydroxide ions (OH⁻) than hydrogen ions (H⁺).

Analyze the Options

Review each option and understand the nature of the salts formed: (1) Salt from a strong acid and strong base results in a neutral solution. (2) Salt from a strong acid and weak base results in an acidic solution. (3) Salt from a weak acid and weak base can be neutral, acidic, or basic depending on the relative strengths of the acid and base. (4) Salt from a weak acid and strong base results in an alkaline solution.

Select the Correct Option

Based on the analysis, the salt made from a weak acid and a strong base (option 4) will lead to an alkaline solution because the conjugate base of the weak acid will produce OH⁻ ions.

Key concepts

acid-base reactions

An **acid-base reaction** is a chemical reaction that occurs between an acid and a base. The result of this reaction is often the formation of water and a salt. An acid donates a proton (H⁺) in the reaction, while a base accepts the proton. These reactions are important in many biological processes and industrial applications. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), they produce water (H₂O) and sodium chloride (NaCl) – a neutral salt.

Types of acid-base reactions include:

• Neutralization: Acid + Base → Salt + Water
• Acid with a carbonate: Acid + Carbonate → Salt + Water + CO₂

Understanding these reactions helps explain why certain salts result in acidic or alkaline solutions in water.

pH levels

The **pH level** measures how acidic or basic (alkaline) a solution is, ranging from 0 to 14. A pH of 7 is neutral, meaning the concentration of hydrogen ions (H⁺) equals the concentration of hydroxide ions (OH⁻).

A solution with a pH less than 7 is acidic:

• Examples include lemon juice and vinegar.
• They have a higher concentration of H⁺ ions.

A solution with a pH greater than 7 is basic or alkaline:

• Examples include baking soda and detergents.
• They have a higher concentration of OH⁻ ions.

In the given exercise, an alkaline solution has a pH greater than 7, indicating it has more OH⁻ ions than H⁺ ions. This helps to determine that the salt causing the solution to be alkaline must come from a weak acid and a strong base.

salt hydrolysis

Hydrolysis is a process where a salt reacts with water to form an acidic or basic solution. The type of solution formed depends on the strengths of the acid and base that created the salt. When a salt dissolves in water, it can either donate H⁺ or OH⁻ ions, influencing the pH.

**Non-reactive salts** from strong acids and bases don’t affect the pH, resulting in a neutral solution.

Salts from:

• Strong acid and weak base release H⁺ ions, making the solution acidic.
• Weak acid and strong base release OH⁻ ions, making the solution basic or alkaline.

This helps to determine in the exercise why the salt from a weak acid and strong base makes the solution alkaline. The weak acid's conjugate base interacts with water to produce OH⁻ ions.

strong and weak acids and bases

An acid or base’s strength is determined by how completely it ionizes in water:

**Strong acids** completely dissociate in water, releasing all their H⁺ ions. Examples include HCl and H₂SO₄.

**Weak acids** only partially dissociate, leaving some of the acid intact. Examples include acetic acid (CH₃COOH) and citric acid.

**Strong bases** fully dissociate to release OH⁻ ions. Examples are NaOH and KOH.

**Weak bases** only partially produce OH⁻ ions in water, such as ammonia (NH₃).

In the exercise, the salt was formed from a weak acid and a strong base, resulting in an alkaline solution. This is because the weak acid’s conjugate base is strong enough to react with water, producing additional OH⁻ ions and raising the pH above 7.