Question

Each of the observations listed was made on a different solution. Given the observations, state which ion studicd in this cxperiment is prescnt. If the test is not definitive, indicate that with a question mark. A. Addition of \(6 \mathrm{M} \mathrm{NaOH}\) and \(\mathrm{Al}\) to the solution produces a vapor that turns red litmus blue. Ion prescnt: B. Addition of \(6 \mathrm{M}\) HCl produces a vapor with an acrid odor. Ion present: C. Addition of \(6 \mathrm{M}\) HCl produces an effervescence. Ion present: D. Addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) plus \(0.1 \mathrm{M} \mathrm{AgNO}_{3}\) produces a precipitate. Ion prescnt: E. Addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) plus \(1 \mathrm{M} \mathrm{BaCl}_{2}\) produces a precipitate. Ion present: F. Addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) plus \(0.5 \mathrm{M}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{MoO}_{4}\) produces a precipitate. Ion present:

Short answer

A. Ammonium ion, \(\mathrm{NH}_{4}^{+}\). B. Chloride ion, \(\mathrm{Cl}^{-}\). C. Carbonate ion, \(\mathrm{CO}_{3}^{2-}\). D. Iodide ion, \(\mathrm{I}^{-}\). E. Sulfate ion, \(\mathrm{SO}_{4}^{2-}\). F. Phosphate ion, \(\mathrm{PO}_{4}^{3-}\).

Step-by-step solution

Observation A

When \(6 \mathrm{M} \mathrm{NaOH}\) and \(\mathrm{Al}\) are added to the solution, a vapor that turns red litmus blue is produced. This suggests that the vapor produced is a basic gas. The most likely gas to be produced in this experiment with these reactants is ammonia gas, \(\mathrm{NH}_{3}\). Therefore, the ion present should be ammonium ion, \(\mathrm{NH}_{4}^{+}\).

Observation B

The addition of \(6 \mathrm{M}\) HCl produces a vapor with an acrid odor, which suggests that the vapor is likely chlorine gas, \(\mathrm{Cl}_{2}\). This means that the ion present in the solution should be the chloride ion, \(\mathrm{Cl}^{-}\).

Observation C

When \(6 \mathrm{M}\) HCl is added to the solution, effervescence is observed, which means a gas is released within the course of the reaction. In this case, the gas released is very likely to be carbon dioxide gas, \(\mathrm{CO}_{2}\), upon reaction with the carbonate ion. Thus, the ion present in the solution should be the carbonate ion, \(\mathrm{CO}_{3}^{2-}\).

Observation D

The addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) and \(0.1 \mathrm{M}\mathrm{AgNO}_{3}\) to the solution results in a precipitate forming. This precipitation suggests the presence of an anion that would form an insoluble salt with silver ions, \(\mathrm{Ag}^{+}\). The most likely ion in this case is the iodide ion, \(\mathrm{I}^{-}\), due to the formation of silver iodide, \(\mathrm{AgI}\), as a precipitate.

Observation E

In this observation, the addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) plus \(1 \mathrm{M}\mathrm{BaCl}_{2}\) produces the precipitation. Barium chloride can react with sulfate ions to form a precipitate of barium sulfate, \(\mathrm{BaSO}_{4}\). Thus, the ion present in this observation is the sulfate ion, \(\mathrm{SO}_{4}^{2-}\).

Observation F

With the addition of \(6 \mathrm{M} \mathrm{HNO}_{3}\) plus \(0.5\mathrm{M}\left(\mathrm{NH}_{4}\right)_{2}\mathrm{MoO}_{4}\), a precipitate forms. This reaction suggests the presence of an ion that can react with the ammonium molybdate to form a precipitate. In this case, the ion present is most likely the phosphate ion, \(\mathrm{PO}_{4}^{3-}\), as ammonium phosphomolybdate forms a precipitate. In summary, the ions present are as follows: A. Ammonium ion, \(\mathrm{NH}_{4}^{+}\). B. Chloride ion, \(\mathrm{Cl}^{-}\). C. Carbonate ion, \(\mathrm{CO}_{3}^{2-}\). D. Iodide ion, \(\mathrm{I}^{-}\). E. Sulfate ion, \(\mathrm{SO}_{4}^{2-}\). F. Phosphate ion, \(\mathrm{PO}_{4}^{3-}\).

Key concepts

Ion Identification

Ion identification is a fundamental aspect of qualitative analysis in chemistry. It involves determining the ions present in a solution based on specific reactions and observations. These ions can be identified through various tests that result in distinct reactions such as color change, gas evolution, or precipitate formation.

For instance, in a solution suspected of containing ammonium ions, the addition of \(\text{NaOH}\) and aluminum might produce a gas that turns red litmus paper blue, indicating basicity. This is due to the formation of ammonia gas, pointing to the presence of ammonium ions \(\text{NH}_4^+\).

Observations are critical in ion identification as different ions exhibit unique behaviors when subjected to chemical tests. This makes it easier to conclude which specific ion is present in a solution.

Chemical Reactions

Chemical reactions form the basis of qualitative analysis tests as they provide the evidence needed to identify ions. Every chemical reaction occurs with reactants changing into products, which could include gases, solids (precipitates), or other substances knowledgeable researchers can identify.

One example from the exercise is the reaction between \(\text{HCl}\) and a carbonate ion, leading to effervescence due to the release of carbon dioxide gas. This is a clear indication of the presence of carbonate ions \(\text{CO}_3^{2-}\).

Chemical reactions highlight the prowess of chemistry in identifying substances non-destructively and sometimes even visually. These reactions are the toolset through which chemists can dissect the ionic composition of complex mixtures.

Precipitation Reactions

Precipitation reactions are a class of reactions where two solutions combine to form an insoluble solid called a precipitate. These are particularly important in ion identification as they can conclusively indicate the presence of specific ions.

For example, in the presence of iodide ions, adding \(\text{AgNO}_3\) results in the formation of a yellow precipitate of silver iodide, \(\text{AgI}\). This is a definitive test for iodide ions \(\text{I}^-\).

These reactions help chemists visually observe the change, and the formation of a precipitate serves as a fingerprint for certain ion combinations. Understanding which substances will precipitate under given conditions is therefore crucial to performing qualitative analysis.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between substances. They are a fundamental type of reaction used in qualitative analysis not only to identify ions but also for their effects such as change in pH or gas release.

An example from our context is the reaction of \(\text{HCl}\) producing an acrid odor due to the generation of chlorine gas, which helps identify chloride ions \(\text{Cl}^-\).

Moreover, acid-base reactions can also cause indicators to change color. For example, if a litmus paper's color changes, it indicates the solution's pH has shifted, suggesting the presence of an acid or a base. These reactions utilize the essence of the acid and base properties to efficiently determine ion presence in solutions.