Problem 28 - General Chemistry

Question

If one mole of the following compounds were each placed into separate beakers containing the same amount of water, rank the \(\mathrm{Cl}^{-}(a q)\) concentrations from highest to lowest (some may be equivalent): \(\mathrm{KCl}, \mathrm{AlCl}_{3}, \mathrm{PbCl}_{2}, \mathrm{NaCl}, \mathrm{HCl}, \mathrm{NH}_{3}, \mathrm{KOH}\), and \(\mathrm{HCN}\).

Step-by-Step Solution

Answer

AlCl₃ > PbCl₂ > KCl = NaCl = HCl > NH₃ = KOH = HCN.

Step 1: Identify Ionic Compounds

First, identify which of the given compounds will dissociate into ions in water. These are typically ionic compounds. The ionic compounds from the list are: \( \text{KCl} \), \( \text{AlCl}_3 \), \( \text{PbCl}_2 \), and \( \text{NaCl} \), and \( \text{HCl} \). These compounds will release \( \text{Cl}^- \) ions into the solution.

Step 2: Determine Moles of \( \text{Cl}^- \) Released

Calculate the number of \( \text{Cl}^- \) ions each compound will produce per mole:- \( \text{KCl} \): 1 mole of \( \text{Cl}^- \)- \( \text{AlCl}_3 \): 3 moles of \( \text{Cl}^- \)- \( \text{PbCl}_2 \): 2 moles of \( \text{Cl}^- \)- \( \text{NaCl} \): 1 mole of \( \text{Cl}^- \)- \( \text{HCl} \): 1 mole of \( \text{Cl}^- \).

Step 3: Evaluate Non-Ionic Compounds

Identify compounds that do not significantly contribute \( \text{Cl}^- \) ions. These primarily involve \( \text{NH}_3 \), \( \text{KOH} \), and \( \text{HCN} \), which do not dissociate into \( \text{Cl}^- \) ions.

Step 4: Rank \( \text{Cl}^- \) Concentrations

Based on the dissociation analysis, rank the compounds by the concentration of \( \text{Cl}^- \) ions:1. \( \text{AlCl}_3 \) (3 moles of \( \text{Cl}^- \))2. \( \text{PbCl}_2 \) (2 moles of \( \text{Cl}^- \))3. \( \text{KCl} \), \( \text{NaCl} \), \( \text{HCl} \) (1 mole each of \( \text{Cl}^- \))4. \( \text{NH}_3 \), \( \text{KOH} \), \( \text{HCN} \) (0 moles of \( \text{Cl}^- \)).

Key Concepts

Dissociation in Water

When ionic compounds dissolve in water, they separate into their individual ions. This process is known as dissociation. It is essential to understand dissociation because it affects the concentration of ions, such as the chloride ions (\(\mathrm{Cl}^{-}\)).
For example, when potassium chloride (\(\mathrm{KCl}\)) is dissolved, it separates into potassium ions and chloride ions:

• \(\mathrm{KCl(aq)} \rightarrow \mathrm{K}^{+}(aq) + \mathrm{Cl}^{-}(aq)\)

This process determines the availability of chloride ions in the solution and impacts various chemical reactions that occur in aqueous solutions.

Chloride Ion Concentration

The concentration of chloride ions (\(\mathrm{Cl}^{-}\)) depends on the degree to which an ionic compound dissociates in water. When comparing different compounds, each can release a different number of chloride ions. For instance:

• \(\mathrm{AlCl}_{3}\) releases three chloride ions because it splits into three \(\mathrm{Cl}^{-}\) ions per molecule.
• \(\mathrm{PbCl}_{2}\) yields two chloride ions per molecule.
• \(\mathrm{NaCl}\) and \(\mathrm{KCl}\) only release one chloride ion each.

The total chloride ion concentration in solution impacts chemical reactions and properties like conductivity and acidity.

Chemical Reactions

Chemical reactions in aqueous solutions often rely on the ions present from dissolved compounds. These ions can combine to form new products or influence the pH of the solution. For example:

• In acid-base reactions, chloride ions are spectators and do not participate.
• In precipitation reactions, if a solution contains \(\mathrm{Ag}^{+}\) ions with \(\mathrm{Cl}^{-}\) ions, silver chloride (\(\mathrm{AgCl}\)) can precipitate out as a solid.

Understanding the role of ions and their concentrations is crucial in predicting the outcome of chemical reactions in solutions.

Moles and Stoichiometry

In chemistry, moles and stoichiometry are vital for calculating the quantities involved in chemical reactions. Stoichiometry helps us understand how reactants transform into products using balanced equations. For example, determining how many moles of chloride ions are produced from certain substances involves knowing how they dissociate.
A compound like \(\mathrm{AlCl}_{3}\) dissociates to provide three moles of \(\mathrm{Cl}^{-}\) for every mole of \(\mathrm{AlCl}_{3}\) added.
This principle allows chemists to predict the yield of a reaction or the concentration of solutions prepared in laboratories.