Problem 117 - Chemistry and Chemical Reactivity

Question

A You have three solutions labeled \(A, B,\) and \(C .\) You know only that each contains a different cation-Na \(^{+}\) \(\mathrm{NH}_{4}^{+},\) or \(\mathrm{H}^{+}\). Each has an anion that does not contribute to the solution \(\mathrm{pH}\) (e.g., \(\mathrm{Cl}^{-}\) ). You also have two other solutions, Y and Z, each containing a different anion, \(\mathrm{Cl}^{-}\) or \(\mathrm{OH}^{-},\) with a cation that does not influence solution \(\mathrm{pH}\left(\text { e.g. }, \mathrm{K}^{+}\right) .\) If equal amounts of \(\mathrm{B}\) and \(\mathrm{Y}\) are mixed, the result is an acidic solution. Mixing A and Z gives a neutral solution, whereas \(\mathrm{B}\) and \(\mathrm{Z}\) give a basic solution. Identify the five unknown solutions. (Adapted from D. H. Barouch: Voyages in Conceptual Chemistry, Boston, Jones and Bartlett, \(1997 .)\) (TABLE CAN'T COPY)

Step-by-Step Solution

Answer

A contains NH_4^+, B contains H^+, C contains Na^+; Y contains Cl^-, Z contains OH^-.

Step 1: Analyzing the Reaction of B and Y

When solutions B and Y are mixed, the resulting solution is acidic. Since Y contains an anion that could be either Cl^- or OH^-, and an acidic solution forms, Y must contain Cl^- because mixing with an OH^- would not likely create an acidic solution. Thus, solution B must contain the cation H^+ to result in an acidic solution when combined.

Step 2: Analyzing the Reaction of A and Z

Mixing solutions A and Z results in a neutral solution. A neutral solution forms with anion OH^- present in Z when solution A contains NH_4^+, as NH_4OH can decompose to produce water and NH_3, both of which do not alter the pH significantly. Therefore, Z must contain OH^- and A contains NH_4^+.

Step 3: Analyzing the Reaction of B and Z

When solutions B and Z are mixed, a basic solution results. We previously concluded that B contains H^+ and Z contains OH^-, which means the production of water is the net result here. Thus, unless there's an error in previous steps, this confirms Z has OH^-.

Step 4: Assigning Remaining Ions

Considering A must contain NH_4^+ and B contains H^+, solution C must then contain Na^+. Since Z has OH^-, solution Y, mixing with B for acidity, contains Cl^-.

Key Concepts

Cation and Anion Reactions

To identify unknown solutions, understanding how cations and anions interact is crucial. Each molecule in a solution consists of positively charged ions, known as cations, and negatively charged ions, referred to as anions.
When solutions are mixed, these ions can recombine to form new compounds, which may influence the properties of the resulting solution.

• **Example:** When you mix solution B (with the cation H⁺) and Y (with the anion Cl^-), they form HCl, which is an acidic compound.
• The presence of specific cations like H⁺ can directly lead to acidic behavior when paired with suitable anions.

Anionic choices such as OH^- are significant too. Mix an anion like OH^- with cations such as NH₄⁺, and a neutral water and NH₃ are produced.
This understanding of how cations such as Na⁺, NH₄⁺, and H⁺ react with anions like Cl^- and OH^- is essential in solution identification.

Acidic, Basic, and Neutral Solutions

Once you know the ions involved, you can predict the nature of the solution—whether it is acidic, basic, or neutral.
Here's how you can assess these possibilities:

• **Acidic Solutions:** When B and Y mix to create an acidic solution, this indicates the presence of H⁺ ions. These ions are known for releasing free hydrogen ions into solution, lowering the pH.
• **Basic Solutions:** B and Z mixing results in a basic solution, which suggests Z contains OH^- ions. When OH^- ions are present, they increase the hydroxide ion concentration, raising the pH.
• **Neutral Solutions:** A and Z form a neutral solution, meaning any excess ions likely neutralized each other. Mixing NH₄⁺ with OH^-, for instance, can lead to water formation, hinting at a balanced pH.

Using these concepts, the behavior of mixtures can be predicted and understood, providing critical clues to identifying unknowns in experiments.

Chemical Reasoning and Analysis

Chemical reasoning involves using observations from experiments to draw logical conclusions about the identities and properties of substances.
This type of analysis is sometimes like a puzzle that relies on several factors:

• **Observation Analysis:** By observing the type of solution formed (acidic, basic, or neutral), inferences about the ions present can be made. Each reaction step provides pivotal information.
• **Sequential Deductions:** Starting from known reactions (e.g., A with Z) and using logical deductions, solutions can be identified systematically. For instance, we concluded Z had OH^- based on its ability to neutralize potential acidity and yield neutrality.
• **Cross-verification:** Validating steps such as reconfirming basic outcomes (B and Z) helps ensure consistency and accuracy in concluding the exact constituents of unknown solutions.

Overall, chemical reasoning combines analytical skills and foundational chemistry knowledge to identify unknown substances efficiently.