Question

A sample may contain any or all of the following ions: \(\mathrm{Hg}_{2}^{2+}\) \(\mathrm{Ba}^{2+},\) and \(\mathrm{Mn}^{2+}\) a. No precipitate formed when an aqueous solution of \(\mathrm{NaCl}\) was added to the sample solution. b. No precipitate formed when an aqueous solution of \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) was added to the sample solution. c. A precipitate formed when the sample solution was made basic with NaOH. Which ion or ions are present in the sample solution?

Short answer

Based on the reactions with NaCl, Na₂SO₄, and NaOH, we can conclude that the sample solution contains only the Mn²⁺ ion.

Step-by-step solution

Investigate the reactions with NaCl

First, consider how each ion reacts with NaCl: 1. Hg₂²⁺ + 2Cl⁻ → Hg₂Cl₂ (white precipitate formed) 2. Ba²⁺ + 2Cl⁻ → BaCl₂ (no precipitate formed) 3. Mn²⁺ + 2Cl⁻ → MnCl₂ (no precipitate formed) Since no precipitate was formed when NaCl was added, we can conclude that Hg₂²⁺ is not present in the sample solution.

Investigate the reactions with Na₂SO₄

Now, consider how the ions react with Na₂SO₄: 1. Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate formed) 2. Mn²⁺ + SO₄²⁻ → MnSO₄ (no precipitate formed) Since no precipitate was formed when Na₂SO₄ was added, we can conclude that Ba²⁺ is not present in the sample solution.

Investigate the reactions with NaOH

Finally, consider how the ions react with NaOH: 1. Mn²⁺ + 2OH⁻ → Mn(OH)₂ (white precipitate formed) A precipitate was formed when the sample solution was made basic with NaOH. Since Mn²⁺ is the only remaining ion that could cause this reaction, we can conclude that Mn²⁺ is present in the sample solution.

Conclusion

Based on the given information and our analysis, we can conclude that the sample solution contains only the Mn²⁺ ion.

Key concepts

Precipitation Reactions

Precipitation reactions are a staple in the study of chemistry, often serving as a visual representation of chemical interactions. In simple terms, when two soluble salts (ionic compounds) react in solution, the formation of an insoluble product, known as a precipitate, may occur. These reactions are not only critical for theoretical study but also have practical applications in areas such as water treatment and qualitative chemical analysis.

For example, when an aqueous solution of sodium chloride (NaCl) is added to a mix of various ions, if a precipitate is observed, it indicates the presence of specific ions that form insoluble chlorides. However, as noted in the step-by-step solution, no precipitate was observed with NaCl, providing important information about the ions present in the solution.

Solubility Rules

Solubility rules are essentially a set of guidelines that help predict the solubility of various ionic compounds in water. Understanding these rules enables students and chemists to foresee which combinations of ions will result in a precipitate and which will remain dissolved. For instance, most chloride salts are soluble in water, except for those containing silver, mercury, or lead cations. Conversely, many sulfate salts are soluble, with barium sulfate being a notable exception.

To apply these rules effectively, as seen in the exercise, knowing that neither NaCl nor Na₂SO₄ caused a precipitate allows the elimination of certain ions from consideration. This process of elimination, guided by solubility rules, is crucial for identifying unknown substances in a mixture. The solution's analysis through these rules narrowed down the possible ions in the sample.

Chemical Identification

The process of chemical identification involves determining the composition of a substance or mixture. Techniques vary from simple, visual tests to complex instrumental methods. In our exercise, the chemical identification was achieved by systematic testing with various reagents, observing for precipitation reactions, and using solubility rules as a reference. The absence or presence of a precipitate upon adding specific compounds can pinpoint the exact ions present in a mixture.

Moreover, the individual characteristics of precipitates, like color and texture, can further affirm the identity of the ions. For instance, when NaOH was added and a white precipitate formed, this aligns with the known reaction of Mn²⁺ with hydroxide ions to form manganese(II) hydroxide. Such methodical analysis allows for a precise identification of the Mn²⁺ ion in the original sample, showcasing how qualitative analysis effectively decodes a mixture's composition without the use of sophisticated instruments.