Problem 20 - Chemistry: The Central Science

Question

Acetone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), is a nonelectrolyte; hypochlorous acid, \(\mathrm{HClO}\), is a weak electrolyte; and ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl}\), is a strong electrolyte. (a) What are the solutes present in aqueous solutions of each compound? (b) If \(0.1 \mathrm{~mol}\) of each compound is dissolved in solution, which one contains \(0.2 \mathrm{~mol}\) of solute particles, which contains \(0.1 \mathrm{~mol}\) of solute particles, and which contains somewhere between \(0.1\) and \(0.2 \mathrm{~mol}\) of solute particles?

Step-by-Step Solution

Answer

(a) The solutes present in aqueous solutions are: 1. Acetone: \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) 2. Hypochlorous Acid: \(\mathrm{H^+}\) and \(\mathrm{ClO^-}\) 3. Ammonium Chloride: \(\mathrm{NH}_{4}^+\) and \(\mathrm{Cl}^-\). (b) Based on the number of solute particles, the solutions are: 1. Acetone has 0.1 mol of solute particles. 2. Hypochlorous acid has between 0.1 and 0.2 mol of solute particles. 3. Ammonium chloride has 0.2 mol of solute particles.

Step 1: Identify Solutes in Aqueous Solutions

To identify the solutes in aqueous solution, we should know how these compounds dissociate when dissolved in water: 1. Acetone, being a nonelectrolyte, does not dissociate. It remains intact: \[\mathrm{CH}_{3} \mathrm{COCH}_{3}\]. 2. Hypochlorous acid, being a weak electrolyte, dissociates partially into ions: \[\mathrm{HClO} \rightleftharpoons \mathrm{H^+} + \mathrm{ClO^-}\]. 3. Ammonium chloride, being a strong electrolyte, dissociates completely into ions: \[\mathrm{NH}_{4} \mathrm{Cl} \rightarrow \mathrm{NH}_{4}^+ + \mathrm{Cl}^-\].

Step 2: Determine the Amount of Solute Particles

Now let's determine the amount of solute particles present when 0.1 mol of each compound is dissolved in a solution: 1. Acetone, 0.1 mol remains intact, so the number of solute particles is 0.1 mol. 2. Hypochlorous acid, 0.1 mol dissociates partially: We assume a certain extent 'x' dissociates, then we have x mol of \(\mathrm{H^+}\) and x mol of \(\mathrm{ClO^-}\). Hence, the number of solute particles is \((0.1 - x) + x + x = 0.1 + x\), where \(0 < x < 0.1\). 3. Ammonium chloride, 0.1 mol dissociates completely, so we have 0.1 mol of \(\mathrm{NH}_{4}^+\) and 0.1 mol of \(\mathrm{Cl}^-\), which gives a total of 0.2 mol solute particles.

Step 3: Identify the Solutions in Terms of Solute Particles

Now we can determine which solution has 0.2 mol, 0.1 mol, and somewhere between 0.1 to 0.2 mol of solute particles: 1. Acetone solution contains 0.1 mol of solute particles. 2. Hypochlorous acid solution contains between 0.1 and 0.2 mol of solute particles. 3. Ammonium chloride solution contains 0.2 mol of solute particles. So, the solution to the exercise is: (a) The solutes present in aqueous solutions are: 1. Acetone: \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) 2. Hypochlorous Acid: \(\mathrm{H^+}\) and \(\mathrm{ClO^-}\) 3. Ammonium Chloride: \(\mathrm{NH}_{4}^+\) and \(\mathrm{Cl}^-\). (b) Based on the number of solute particles, the solutions are: 1. Acetone has 0.1 mol of solute particles. 2. Hypochlorous acid has between 0.1 and 0.2 mol of solute particles. 3. Ammonium chloride has 0.2 mol of solute particles.

Key Concepts

Nonelectrolytes

Nonelectrolytes are substances that, when dissolved in water, do not produce ions. These materials do not conduct electricity in their aqueous form because they do not separate into charged particles. A classic example of a nonelectrolyte is acetone (CH_{3}COCH_{3}), which is used in the textbook exercise. Acetone molecules remain whole in solution, resulting in 0.1 mol of solute particles since it does not dissociate at all.

Understanding this concept is crucial when predicting the behavior of a substance in solution. For students, a helpful way to remember nonelectrolytes is to think about many organic compounds, such as sugars and alcohols, which typically fall into this category.

Weak Electrolytes

Weak electrolytes are substances that partially dissociate into ions when dissolved in water, which means they only partially conduct electricity. Hypochlorous acid (HClO) demonstrates this behavior. In the exercise, the key is to realize that weak electrolytes establish a dynamic equilibrium between the undissociated molecules and the ions they form.

The proportion of dissociated molecules to undissociated ones varies, which is represented by 'x' in the solution; 'x' being the fraction that dissociates. Thus, the total amount of solute particles is greater than 0.1 mol but less than 0.2 mol. To help students visualize this concept, using an analogy like a partially unzippered zipper, where some teeth separate and others remain joined, could be instructive.

Strong Electrolytes

Strong electrolytes are substances that fully dissociate into ions in solution, thus conducting electricity well. Ammonium chloride (NH_{4}Cl), as used in our problem, is a strong electrolyte. When 0.1 mol of ammonium chloride is dissolved in water, it fully separates into 0.1 mol of ammonium ions (NH_{4}^{+}) and 0.1 mol of chloride ions (Cl^{-}), totaling 0.2 mol of ions.

To assimilate this knowledge, students might imagine a strong electrolyte as a bag of popcorn in a microwave; once heated (dissolved in water), all kernels (molecules) pop (dissociate) into popcorn (ions). This full dissociation characteristic of strong electrolytes is critical for understanding their behavior in various chemical reactions and applications.