Question

A student is asked to identify the metal nitrate present in an aqueous solution. The cation in the solution can be either \(\mathrm{Na}^{+}, \mathrm{Ba}^{2+}, \mathrm{Ag}^{+},\) or \(\mathrm{Ni}^{2+} .\) Results of solubility experiments are as follows: unknown \(+\) chloride ions-no precipitate unknown + carbonate ions-precipitate unknown \(+\) sulfate ions-precipitate What is the cation in the solution?

Short answer

Question: Determine the cation present in an unknown aqueous solution of a metal nitrate, given that the cation forms a precipitate with carbonate and sulfate ions but not chloride ions. Answer: The cation present in the unknown aqueous solution is Ba²⁺, and the metal nitrate is Ba(NO₃)₂ (barium nitrate).

Step-by-step solution

Write down known solubility rules

The solubility rules are as follows: 1. Most nitrate salts are soluble. 2. Most salts containing alkali metal ions (Group 1 elements) are soluble. 3. Most chloride, bromide, and iodide salts are soluble, except for salts containing silver, mercury(I), and lead(II). 4. Most sulfate salts are soluble, with the exception of calcium sulfate, barium sulfate, and lead(II) sulfate. 5. Most carbonate salts are insoluble, except for salts containing alkali metal ions and ammonium ions.

Analyze the given results with solubility rules

We are given that the unknown cation forms: - No precipitate with chloride ions: This means that the cation is not silver, since silver chloride is insoluble. So, we can rule out \(\mathrm{Ag}^+\). - A precipitate with carbonate ions: This means that the cation is not an alkali metal, since alkali metal carbonates are soluble. We can rule out \(\mathrm{Na}^+\). - A precipitate with sulfate ions: This means that the cation forms an insoluble sulfate, which could be \(\mathrm{Ba}^{2+}\) or \(\mathrm{Ni}^{2+}\), as barium sulfate and nickel(II) sulfate are insoluble.

Determine the cation in the solution

Based on the analysis in Step 2, we have narrowed down the possible cations to \(\mathrm{Ba}^{2+}\) and \(\mathrm{Ni}^{2+}\). However, nickel(II) chloride is soluble, which would not result in a precipitate when added to a solution containing chloride ions. Thus, the cation in the solution must be \(\mathrm{Ba}^{2+}\). Therefore, the metal nitrate present in the aqueous solution is barium nitrate (\(\mathrm{Ba(NO_3)_2}\)).

Key concepts

Cation Identification

Understanding cation identification is essential in the field of chemistry as it allows us to determine the positive ion present in a compound. This process is vital, especially when dealing with aqueous solutions, where various ions are free to interact.

In the given exercise, the student's task is to identify a cation in an aqueous solution through conducting solubility experiments. Not all cations behave the same when mixed with different anions; hence, we use specific rules and reactions like those involving chloride, carbonate, and sulfate ions to identify our unknown cation.

Through the process of elimination and by observing whether a precipitate forms or not, the student can determine which cation is present. If no precipitate forms with chloride ions, we can eliminate cations that are known to form insoluble chlorides. Similarly, if a precipitate forms with sulfate and carbonate ions, we can conclude that the cation forms insoluble compounds with these anions. Identifying cations with such systematic experimentation is a fundamental skill in analytical chemistry.

Precipitation Reactions

A precipitation reaction occurs when two aqueous solutions combine to form a solid, insoluble product known as a precipitate. This process is a result of the interaction between cations and anions, where the products' solubility is low enough that they form a solid compound.

In our exercise, we observe two instances of precipitate formation: when the unknown cation reacts with carbonate ions, and sulfate ions. These observations are crucial as they correspond to specific solubility rules, helping us deduce possible cation identities. For example, since most carbonate salts are insoluble, except those with alkali metal ions and ammonium ions, the formation of a precipitate with carbonate ions suggests that the cation is neither.

This understanding of precipitation reactions is not only useful for cation identification but also in broader applications such as water treatment, where certain contaminants are removed by inducing precipitation.

Aqueous Solution Analysis

Analyzing aqueous solutions involves more than just identifying the substances dissolved in water. It is about understanding the nature of the interactions among solutes and using qualitative data, such as precipitate formation, to deduce quantitative facts.

The exercise presents a classic example of aqueous solution analysis. By applying known solubility rules, which dictate the solubility of various ionic compounds in water, we can make logical connections between observed reactions and possible substances in the solution. The absence or presence of a precipitate in reactions with chloride, carbonate, and sulfate ions allows us to systematically determine the identity of the unknown cation.

Mastering aqueous solution analysis is a key competency for students, as it develops the foundational skill of making informed predictions based on empirical evidence, a methodology that is at the core of scientific inquiry.