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Ch.4 - Reactions in Aqueous Solution
All textbooksBrown 14th EditionCh.4 - Reactions in Aqueous SolutionProblem 93
Chapter 4, Problem 93

Suppose you have a solution that might contain any or all of the following cations: Ni2+, Ag+, Sr2+, and Mn2+. Addition of HCl solution causes a precipitate to form. After filtering off the precipitate, H2SO4 solution is added to the resulting solution and another precipitate forms. This is filtered off, and a solution of NaOH is added to the resulting solution. No precipitate is observed. Which of the four ions listed above must be absent from the original solution?

Verified step by step guidance
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Step 1: Identify the cations that form precipitates with HCl. Ag+ forms a precipitate with Cl- as AgCl, which is insoluble in water. Therefore, if a precipitate forms upon adding HCl, Ag+ is present.
Step 2: Consider the cations that form precipitates with H2SO4. Sr2+ forms a precipitate with SO4^2- as SrSO4, which is insoluble in water. If a precipitate forms upon adding H2SO4, Sr2+ is present.
Step 3: Analyze the cations that form precipitates with NaOH. Ni2+ and Mn2+ can form precipitates with OH- as Ni(OH)2 and Mn(OH)2, respectively, which are insoluble in water. If no precipitate forms upon adding NaOH, both Ni2+ and Mn2+ must be absent.
Step 4: Conclude which ions must be absent. Since no precipitate forms with NaOH, both Ni2+ and Mn2+ are absent from the original solution.
Step 5: Summarize the findings. The absence of a precipitate with NaOH indicates that Ni2+ and Mn2+ are not present in the original solution.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react to form an insoluble compound, known as a precipitate. In this scenario, the addition of HCl leads to the formation of a precipitate, indicating that at least one of the cations present forms an insoluble chloride. Understanding which cations can form precipitates with HCl is crucial for identifying the ions in the original solution.
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Selective Precipitation

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve in water. For example, most chlorides are soluble except for those of Ag+, Pb2+, and Hg2^2+. Knowing these rules allows us to determine which cations can remain in solution after the addition of HCl and which will precipitate, aiding in the identification of the absent ions.
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Complex Ion Formation

Complex ion formation occurs when metal cations bond with ligands, often affecting their solubility. In this case, the addition of NaOH does not produce a precipitate, suggesting that any cation that forms a soluble complex with hydroxide ions is likely absent from the original solution. This concept is essential for understanding the behavior of the remaining ions after the sequential addition of reagents.
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Related Practice
Textbook Question

Uranium hexafluoride, UF6, is processed to produce fuel for nuclear reactors and nuclear weapons. UF6 can be produced in a two-step reaction. Solid uranium (IV) oxide, UO2, is first made to react with hydrofluoric acid (HF) solution to form solid UF4 with water as a by-product. UF4 further reacts with fluorine gas to form UF6. (a) Write the balanced molecular equations for the conversion of UO2 into UF4 and the conversion of UF4 to UF6. (b) Which step is an acid-base reaction?

Textbook Question

The accompanying photo shows the reaction between a solution of Cd(NO3)2 and one of Na2S. (b) What ions remain in solution?

Textbook Question

The accompanying photo shows the reaction between a solution of Cd(NO3)2 and one of Na2S. (d) Is this a redox reaction?

Textbook Question

Antacids are often used to relieve pain and promote healing in the treatment of mild ulcers. Write balanced net ionic equations for the reactions between the aqueous HCl in the stomach and each of the following substances used in various antacids: (a) Al(OH)3(s) (b) Mg(OH)2(s) (c) MgCO3(s) (d) NaAl(CO3)(OH)2(s) (e) CaCO3(s).

Textbook Question

The commercial production of nitric acid involves the following chemical reactions:

4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)

2 NO(g) + O2(g) → 2 NO2(g)

3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g)

(a) Which of these reactions are redox reactions?