Question

An unknown solid is entirely soluble in water. On addition of dilute HCl, a precipitate forms. After the precipitate is filtered off, the \(\mathrm{pH}\) is adjusted to about 1 and \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled in; a precipitate again forms. After filtering off this precipitate, the pH is adjusted to 8 and \(\mathrm{H}_{2} \mathrm{~S}\) is again added; no precipitate forms. No precipitate forms upon addition of \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{HPO}_{4}\). (See Figure 7.23.) The remaining solution shows a yellow color in a flame test (see Figure 7.22). Based on these observations, which of the following compounds might be present, which are definitely present, and which are definitely absent: CdS, \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{HgO}_{3}, \mathrm{ZnSO}_{4}, \mathrm{Cd}\left(\mathrm{NO}_{3}\right)_{2}\), and \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) ?

Short answer

The final conclusions based on the given observations are: - CdS, Cd(NO3)2, and ZnSO4 are definitely absent. - Na2SO4 is definitely present as it produces a yellow flame test. - Pb(NO3)2 and HgO might be present, but we lack sufficient information to narrow it down further.

Step-by-step solution

Identify the possible compounds that may form precipitates upon adding dilute HCl.

When the unknown solid is dissolved in water and dilute HCl is added, a precipitate forms. This suggests that the solid may contain a cation that forms an insoluble chloride salt upon reacting with Cl- ions. Based on solubility rules, Pb²⁺, Hg²⁺, and Cd²⁺ are potential candidates to form insoluble chloride salts. These possibilities match with Pb(NO3)2, HgO, and Cd(NO3)2.

Determine the compounds that may form precipitates after adjusting the pH to 1 and bubbling H2S.

When the pH is adjusted to about 1 and H2S is bubbled into the solution, a precipitate forms again. At a pH around 1, the sulfides of group II cations (like Pb²⁺, Hg²⁺, and Cd²⁺) tend to be insoluble. So, this observation further confirms the possibility that Pb(NO3)2, HgO, or Cd(NO3)2 may be present.

Determine the compounds that may form precipitates after adjusting the pH to 8 and bubbling H2S without forming a precipitate.

When the pH is adjusted to 8 and H2S is added again, no precipitate forms. At a pH around 8, the sulfides of group II cations (like Zn²⁺ and Cd²⁺) tend to be insoluble. However, since no precipitate forms at pH 8, it eliminates the possibility of ZnSO4 and further confirms the absence of Cd²⁺. This means that Pb(NO3)2 and HgO are still viable options, while Cd(NO3)2 and ZnSO4 are definitely absent.

Eliminate options based on the lack of (NH4)2HPO4 precipitate.

Since no precipitate forms upon addition of (NH4)2HPO4, it means that there is no significant presence of cations that would form precipitates with phosphate ions. The given compounds do not contain incompatible cations with phosphate ions. Therefore, this step does not provide further information about the presence of any of the compounds.

Determine the compound based on the yellow color in the flame test.

The flame test shows a yellow color, which corresponds to the presence of Na⁺ ions. This observation confirms the existence of Na2SO4 in the unknown solid. Hence, Na2SO4 is definitely present. The final conclusions based on the observations are: - CdS and Cd(NO3)2 are definitely absent because Cd forms a precipitate at pH 8 with H2S. - ZnSO4 is definitely absent because Zn forms a precipitate at pH 8 with H2S. - Na2SO4 is definitely present as it produces a yellow flame test. - Pb(NO3)2 and HgO might be present, but we lack sufficient information to narrow it down further.

Key concepts

Precipitation Reactions

In the world of chemistry, a precipitation reaction occurs when two soluble substances in a solution react to form an insoluble product, known as a precipitate. These reactions are vital for qualitative analysis, which involves determining the composition of chemical compounds.
When a precipitate forms upon adding dilute HCl to the solution, it indicates the presence of ions that lead to insoluble chloride salts. Consider cations like Pb²⁺, Hg²⁺, and Cd²⁺. These ions can form chlorides that do not dissolve in water. This principle helps pinpoint which compounds are involved in the reaction. For instance, in the above exercise, Cd(NO₃)₂, HgO, and Pb(NO₃)₂ become suspected substances because they may form such insoluble salts with Cl⁻ ions. Understanding precipitation reactions is crucial because detecting the presence or absence of a precipitate involves recognizing which ions are likely to interact based on the solubility rules, giving insights into the component present in the solution.

Flame Test

The flame test is a simple, yet effective analytical procedure in chemistry to identify metal ions based on the color they emit in a flame. When a metal salt is introduced into a flame, the electrons gain energy and jump to higher energy levels.
As they return to their original orbits, they release this energy in the form of light. Different elements emit different colors, making it a powerful tool for element identification. In our exercise, the flame test exhibited a yellow color, a strong indicator of sodium (Na⁺) ions. The distinct bright yellow flame is characteristic of sodium compounds, such as Na₂SO₄ in the exercise. Being able to match flame colors to their respective elements is not only exciting but a straightforward method of identifying compounds in an unknown sample.

Solubility Rules

Solubility rules are essential guidelines that predict the solubility of compounds in water. For learners analyzing chemical solutions, memorizing these rules can simplify the process of predicting precipitation reactions.
These rules build on simple principles, such as salts of alkali metals and ammonium ions (like Na⁺ and NH₄⁺) are often soluble. The rules also indicate that most chlorides, bromides, and iodides are soluble, except those formed with Ag⁺, Pb²⁺, and Hg²⁺. In the given problem, understanding these rules helps us deduce why certain ions precipitate out of solution under specific conditions. For example, adjusting the pH of a solution affects the solubility of certain salts. The sulfides of group II ions form precipitates at low pH levels (<1) while remaining soluble at a relatively more neutral pH (approximately 8), confirming or ruling out potential presence in solutions, as seen with ions like Cd²⁺ under alkaline conditions.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H⁺) between reacting species, significantly altering the pH of solutions and influencing solubility behaviors. In qualitative analysis, adjusting the pH of a solution can aid in identifying substances.
For example, in our exercise, the adjustment of pH to 1 and bubbling hydrogen sulfide (H₂S) leads to the precipitation of sulfides from ions like Pb²⁺, Cd²⁺, and Hg²⁺. This is due to the decrease in solubility of their sulfides at acidic pH levels. When the pH is increased to around 8, the lack of precipitation indicates ions with sulfides soluble at this pH. Comprehending the effects of pH on reactions helps in predicting which substances will precipitate and which remain dissolved, guiding the way to determine the components of unknown solutions accurately.