Question

Assume that you have an aqueous solution of an unknown salt. Treatment of the solution with dilute \(\mathrm{BaCl}_{2}, \mathrm{AgNO}_{3}\), and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) produces no precipitate. Which of the following anions might the solution contain? (a) \(\mathrm{Cl}\) (b) \(\mathrm{NO}_{3}^{-}\) (c) \(\mathrm{OH}^{-}\) (d) \(\mathrm{SO}_{4}{ }^{2-}\)

Short answer

The solution might contain nitrate (NO₃⁻) ions.

Step-by-step solution

Identify Reactants for Precipitation

To determine which anions produce a precipitate, we need to know the common precipitates for given cations. Barium chloride (BaCl₂) typically forms precipitates with sulfate (SO₄²⁻) ions. Silver nitrate (AgNO₃) forms precipitates with chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻) ions. Copper(II) nitrate, Cu(NO₃)₂, potentially forms precipitates with hydroxide ions (OH⁻).

Analyze Observations with BaCl₂

Treatment with BaCl₂ produces no precipitate, which suggests that the unknown salt solution likely does not contain sulfate ions (SO₄²⁻) because BaSO₄ would form a precipitate if present.

Analyze Observations with AgNO₃

Treatment with AgNO₃ produces no precipitate, eliminating the possibility of chloride ions (Cl⁻) because AgCl would form a precipitate if present.

Analyze Observations with Cu(NO₃)₂

Since Cu(NO₃)₂ does not produce a precipitate, hydroxide ions (OH⁻) are unlikely to be present, as Cu(OH)₂ would form a precipitate.

Consider Remaining Anions

The only anion left to consider is nitrate (NO₃⁻). Nitrate does not form precipitates with the tested reagents, which is consistent with the observations. Therefore, the solution may contain nitrate ions.

Key concepts

Precipitation Reactions

Precipitation reactions play an essential role in chemistry, especially when it comes to identifying ions in a solution. These reactions occur when solutions of two soluble salts are mixed, resulting in the formation of an insoluble solid called a precipitate. This solid separates from the solution, creating a visible change that indicates a reaction has occurred.

Several important factors can affect precipitation reactions:

• Solubility Rules: Solubility rules help predict whether a precipitate will form. For example, most sulfates are soluble, but barium sulfate ( BaSO₄) is not and will form a precipitate.
• The type of ions present: Different reagents produce precipitates with specific ions, making it easier to determine the identity of unknown ions through systematic testing.

In the context of the original exercise, special reagents like barium chloride, silver nitrate, and copper(II) nitrate are used due to their known reactivities with particular ions. By observing which reactions occur and which do not, we can determine the presence or absence of specific anions.

Anion Identification

Anion identification is a critical step in analyzing chemical solutions, as it involves determining which negative ions (anions) are present. This typically involves a series of qualitative tests where different reagents are added to the solution to see if a reaction or a precipitate forms.

The key to identifying anions is understanding how they react with various cations. Here's how you can approach anion identification:

• Use of Test Reagents: Certain reagents have a very well-established chemistry in forming precipitates with specific anions. For instance, barium chloride (BaCl₂) is used to test for sulfate ions. Silver nitrate (AgNO₃) helps identify halide ions like chloride.
• Observation of Precipitation: The appearance or absence of a precipitate is a direct indicator of the presence or absence of particular anions. If no precipitate forms in a reaction that typically produces one, the corresponding anion is absent.

In the exercise given, by noting the absence of precipitates with all reagents, we deduced that common reactive anions like chloride, sulfate, and hydroxide are not present, which helped us identify nitrate as the likely remaining anion.

Chemical Reactions in Solutions

Chemical reactions in solutions are a cornerstone of practical chemistry and involve interactions between substances when dissolved in a solvent, typically water. These reactions can manifest visibly as changes in color or the formation of a precipitate. Here are some important factors about reactions in solutions:

• Molecular Interactions: When dissolved in water, ionic compounds disassociate into positive and negative ions. These ions can interact with each other, leading to new combinations and precipitates.
• Predictability of Reactions: By using known reagents and referencing solubility guidelines, chemists can predict reactions that will occur when different solutions are combined. For example, barium ions will react with sulfate ions to form barium sulfate, a typical white precipitate.

The provided exercise demonstrated the importance of these reactions: by systematically applying different reagents, it was possible to determine which ions were not present in the solution and infer which ones were. This methodical approach is central to qualitative chemical analysis, allowing chemists to focus on logic and observation to reach conclusive results.