Question

An aqueous solution of an unknown solute is tested with litmus paper and found to be acidic. The solution is weakly conducting compared with a solution of NaCl of the same concentration. Which of the following substances could the unknown be: KOH, \(\mathrm{NH}_{3}, \mathrm{HNO}_{3}, \mathrm{KClO}_{2}, \mathrm{H}_{3} \mathrm{PO}_{3}, \mathrm{CH}_{3} \mathrm{COCH}_{3}(\) acetone)?

Short answer

The unknown substance is likely to be H₃PO₃ (Phosphorous acid) as it forms an acidic solution and has weak conductivity when dissolved in water, which matches the given description.

Step-by-step solution

Identify acidic substances

First, we'll determine which of the given substances can produce acidic solutions when dissolved in water. Acids donate protons (H⁺) into the solution, resulting in a lowered pH. 1. KOH - This is a strong base, which would produce a basic solution, not acidic. 2. NH₃ - This is a weak base and can't form an acidic solution. 3. HNO₃ - This is a strong acid and can form an acidic solution. 4. KClO₂ - This is a salt, and it doesn't have any acidic protons so it cannot form an acidic solution. 5. H₃PO₃ - This is a weak acid, and it can form an acidic solution. 6. CH₃COCH₃ (acetone) - This is a neutral organic compound, and it does not form acidic solutions. From the above list, HNO₃ and H₃PO₃ are the only substances that can form acidic solutions. Now we need to analyze their conductivity properties.

Compare conductivity between substances

Now let's analyze the conductivity of the remaining two substances: 1. HNO₃: This is a strong acid that dissociates completely in water into H⁺ and NO₃⁻ ions. This results in a high concentration of ions in the solution, which would make it a good conductor. But the unknown solution is weakly conducting, so HNO₃ cannot be the unknown substance. 2. H₃PO₃: This is a weak acid that does not completely dissociate in water. Its dissociation generates a lower concentration of ions in the solution and results in weak conductivity. This matches the description of the unknown substance. Considering both the acidic nature and conductivity, the unknown substance is likely to be: H₃PO₃ (Phosphorous acid)

Key concepts

Aqueous Solution

An aqueous solution is simply a solution where water serves as the solvent. In such solutions, the solute—the substance being dissolved—negotiates with water molecules to form a homogeneous mixture. This process depends heavily on the nature of the solute: whether it's ionic or covalent. Aqueous solutions are integral to many chemical reactions and processes, as water is an excellent solvent, often helping to dissolve a wide range of substances.

• Water acts as a solvent in which solute particles are dispersed.
• Ionic compounds tend to dissociate in aqueous solutions.
• Covalent substances may dissolve without dissociation.

Understanding how different substances behave in aqueous solutions is crucial in predicting a solution's characteristics, such as its acidity or conductivity.

Conductivity

Conductivity in chemistry refers to a solution's ability to conduct electricity. This property is directly linked to the presence of free ions in the solution. When ionic compounds dissolve in water, they separate into charged particles called ions. These ions carry electrical current through the solution.

• High conductivity indicates a high concentration of ions.
• Weakly conducting solutions have fewer ions present.

A weak acid, like phosphorous acid, dissociates marginally, contributing fewer ions compared to strong acids like nitric acid. Thus, the conductivity test becomes a valuable tool for identifying properties of substances in aqueous solutions.

Acid Dissociation

Acid dissociation is the process by which an acid molecule splits into ions when dissolved in water. This process is fundamental to understanding acids' behavior in solutions. Strong acids, such as nitric acid, dissociate completely, resulting in a high number of hydrogen ions, which greatly affect the solution's properties, including its pH and conductivity.

Complete dissociation: Results in strong conductivity.

Partial dissociation: Leads to weak conductivity.

Dissociation is crucial to chemical reactions involving acids.

For weak acids like phosphorous acid, only a fraction of acid molecules dissociate, which explains both its acidic nature and low conductivity in solutions.

Weak Acid

A weak acid is one that doesn't fully dissociate into its ions when in an aqueous solution. This means that the concentration of hydrogen ions ( H⁺ ) in the solution is lower compared to strong acids, leading to a higher pH. Weak acids often serve important roles in buffers, regulating pH in a variety of chemical and biological processes.

• Incomplete dissociation characterizes weak acids.
• Examples include phosphorous acid and acetic acid.
• Important in biological and chemical systems due to buffering properties.

In the context of the exercise, the weak conductivity exhibited by the unknown solution points towards phosphorous acid, a representative weak acid.

Litmus Test

The litmus test is a simple, quick way to determine the acidity or basicity of a solution. Litmus paper is soaked in dyes derived from lichens, which change color according to the pH of the solution they are in. When immersed in an acidic solution, litmus paper will turn red, indicating the presence of hydrogen ions. This test doesn't provide the strength of the acid but confirms its presence.

• Red indicates acidity (pH less than 7).
• Blue indicates basicity (pH greater than 7).

Within the context of the exercise, the litmus test helped identify the unknown solution as acidic, helping narrow down the possibilities to those with acidic characteristics, eventually pointing towards phosphorous acid due to its weak conductivity and acidic nature.