Problem 21 - Chemistry The Central Science In Si Units

Question

Using solubility guidelines, predict whether each of the following compounds is soluble or insoluble in water: (a) \(\mathrm{Hg}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{NH}_{4} \mathrm{OH},\) (c) \(\mathrm{Ni}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\), (d) \(\mathrm{AgNO}_{3}\), (e) \(\mathrm{FeCO}_{3}\).

Step-by-Step Solution

Answer

\( \mathrm{Hg}_2\mathrm{SO}_4 \) is insoluble, \( \mathrm{NH}_4\mathrm{OH} \) is soluble, \( \mathrm{Ni}(\mathrm{CH}_3\mathrm{COO})_2 \) is soluble, \( \mathrm{AgNO}_3 \) is soluble, and \( \mathrm{FeCO}_3 \) is insoluble.

Step 1: Understanding Solubility Guidelines

Solubility guidelines help determine if a compound will dissolve in water. Generally, compounds containing ammonium (\(\mathrm{NH}_4^+\)), nitrates (\(\mathrm{NO}_3^-\)), and acetates (\(\mathrm{CH}_3\mathrm{COO}^-\)) are soluble. Most sulfates (\(\mathrm{SO}_4^{2-}\)) are also soluble, except those with Pb, Ba, Sr, Hg. Carbonates (\(\mathrm{CO}_3^{2-}\)) are insoluble except those with ammonium and alkali metals.

Step 2: Analyzing Compound \( \mathrm{Hg}_2\mathrm{SO}_4 \)

\( \mathrm{Hg}_2\mathrm{SO}_4 \) involves mercury and sulfate. Solubility guidelines state that most sulfates are soluble, but those containing mercury are exceptions. Therefore, \( \mathrm{Hg}_2\mathrm{SO}_4 \) is insoluble in water.

Step 3: Analyzing Compound \( \mathrm{NH}_4\mathrm{OH} \)

Compounds containing ammonium ions (\( \mathrm{NH}_4^+ \)) are soluble in water according to solubility guidelines. Therefore, \( \mathrm{NH}_4\mathrm{OH} \) is soluble in water.

Step 4: Analyzing Compound \( \mathrm{Ni}(\mathrm{CH}_3\mathrm{COO})_2 \)

Acetates (\( \mathrm{CH}_3\mathrm{COO}^- \)) are generally soluble in water. \( \mathrm{Ni}(\mathrm{CH}_3\mathrm{COO})_2 \) involves the acetate ion, so it is soluble in water.

Step 5: Analyzing Compound \( \mathrm{AgNO}_3 \)

Nitrates (\( \mathrm{NO}_3^- \)) are always soluble in water. Therefore, \( \mathrm{AgNO}_3 \) is soluble in water.

Step 6: Analyzing Compound \( \mathrm{FeCO}_3 \)

Carbonates (\( \mathrm{CO}_3^{2-} \)) are generally insoluble, except those of ammonium and group 1 alkali metals. \( \mathrm{FeCO}_3 \) does not contain ammonium or alkali metals, so it is insoluble in water.

Key Concepts

Ammonium Compounds

Ammonium compounds have a special place in solubility guidelines. This is because the ammonium ion \( \mathrm{NH}_4^+ \) tends to dissolve readily in water. Regardless of the anion with which it is paired, compounds containing the ammonium ion are generally soluble.
Ammonium hydroxide, represented as \( \mathrm{NH}_4\mathrm{OH} \), is a classic example of this principle, and while it can act as a weak base in solution, it still dissolves completely given enough ammonium present.
This characteristic makes ammonium compounds quite flexible and useful in various chemical applications, especially where solubility is needed.

Sulfate Exceptions

While most sulfate compounds \( \mathrm{SO}_4^{2-} \) are soluble, certain exceptions exist when paired with particular cations. This generally includes lead (\( \mathrm{Pb}^{2+} \)), barium (\( \mathrm{Ba}^{2+} \)), strontium (\( \mathrm{Sr}^{2+} \)), and mercury (\( \mathrm{Hg}_2^{2+} \)).
These exceptions occur due to the specific chemical structures and interactions, causing the resulting sulfate compounds to precipitate out of solution rather than dissolve.
For example, \( \mathrm{Hg}_2 \mathrm{SO}_4 \), which includes mercury, does not dissolve in water, making it insoluble. Knowing these exceptions can be crucial in predicting solubility outcomes and understanding chemical behavior.

Acetate Solubility

Acetate ions \( \mathrm{CH}_3 \mathrm{COO}^- \) are another example of ions that almost always form soluble compounds. The weak acid origin of acetate, which comes from acetic acid, endows it with a particular stability in aqueous solutions.
Compounds like nickel acetate \( \mathrm{Ni}(\mathrm{CH}_3 \mathrm{COO})_2 \) dissolve in water without exception under typical conditions.
The consistent solubility of acetates makes them predictable and useful for forming aqueous solutions where complete dissolution is desired.

Nitrate Solubility

Nitrates \( \mathrm{NO}_3^- \) are uniquely reliable when it comes to solubility guidelines. They are well-known for their high solubility due to the delocalized electron cloud which stabilizes the nitrate ion across various temperatures and conditions.
Compounds such as silver nitrate \( \mathrm{AgNO}_3 \) illustrate the always-soluble nature of nitrates, making them frequently used in precipitation reactions and as sources of nitrate ions in aqueous chemistry.
The solubility ensures that the nitrate ions are available to participate in a wide array of chemical reactionswithout precipitating out of solution.

Carbonate Insolubility

Carbonate ions \( \mathrm{CO}_3^{2-} \) typically form compounds that are insoluble in water. Exceptions occur when they pair with ammonium or alkali metals from Group 1 of the periodic table, such as sodium (\( \mathrm{Na}^+ \)) or potassium (\( \mathrm{K}^+ \)).
In the case of iron carbonate \( \mathrm{FeCO}_3 \), which does not involve ammonium or alkali metal ions, the compound is not soluble and remains as a precipitated solid in water.
Understanding the insolubility of carbonates can be crucial in identifying where filtered solutions may precipitate or remain clear based on the formation of insoluble ionic compounds.