Problem 22 - Chemistry: The Central Science in SI Units, Global Edition

Question

Predict whether each of the following compounds is solu- ble in water: (a) \(\mathrm{MgS},(\mathbf{b}) \mathrm{Cr}(\mathrm{OH})_{3},(\mathbf{c}) \mathrm{Zn} \mathrm{Cl}_{2}\) (d) \(\mathrm{Pb}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) (e) \(\mathrm{Sr}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\)

Step-by-Step Solution

Answer

The solubility of the given compounds in water is as follows: (a) MgS is soluble, (b) Cr(OH)₃ is insoluble, (c) ZnCl₂ is soluble, (d) Pb₃(PO₄)₂ is insoluble, (e) Sr(CH₃COO)₂ is soluble.

Step 1: Compound (a): MgS

According to Rule 6, most sulfides are insoluble, except those of alkali metals, alkaline earth metals, and ammonium. Magnesium (Mg) is an alkaline earth metal. Therefore, MgS is soluble in water.

Step 2: Compound (b): Cr(OH)₃

According to Rule 5, most hydroxides are insoluble, except those of alkali metals, ammonium, and heavier alkaline earth metals (calcium, strontium, barium). Chromium (Cr) is not an alkali metal, ammonium, or a heavier alkaline earth metal, so Cr(OH)₃ is insoluble in water.

Step 3: Compound (c): ZnCl₂

According to Rule 3, most chloride, bromide, and iodide salts are soluble, except those of silver, lead(II), and mercury(I). Zinc (Zn) is not one of the exceptions, so ZnCl₂ is soluble in water.

Step 4: Compound (d): Pb₃(PO₄)₂

According to Rule 7, most carbonate, chromate, phosphate, and oxalate salts are insoluble, except those of alkali metals and ammonium. Lead (Pb) is not an alkali metal or ammonium, so Pb₃(PO₄)₂ is insoluble in water.

Step 5: Compound (e): Sr(CH₃COO)₂

According to Rule 2, most acetates are soluble. Strontium (Sr) is not an exception, so Sr(CH₃COO)₂ is soluble in water.

Key Concepts

Insoluble Compounds

Many compounds do not dissolve well in water and are classified as insoluble compounds. Insolubility in chemistry refers to substances that do not readily dissolve and remain predominantly as solid in a solvent. Understanding insoluble compounds can be particularly useful in predicting the outcomes of chemical reactions and processes.

There are general rules that provide guidelines on which compounds are typically insoluble:

• Carbonates and Phosphates: Most are insoluble, with exceptions for compounds of alkali metals and ammonium.
• Hydroxides: Generally insoluble except for those involving alkali metals and some alkaline earth metals like calcium, strontium, and barium.
• Sulfides: Insoluble for most combinations except when combined with alkali metals, alkaline earth metals, and ammonium.

When trying to determine whether a compound is insoluble, one should look at its constituent ions and check against known solubility rules. A clear example is chromium hydroxide, Cr(OH) , which is insoluble because it does not meet any of the outlined exceptions.

Soluble Compounds

Soluble compounds are those that dissolve readily in water, forming a uniform solution. In many cases, dissolving is a crucial part of reactions in aqueous environments. Knowing which compounds are soluble aids chemists in predicting the behavior and products of such reactions.

Some rules of thumb include the following, pointing to the likelihood of solubility:

• Sulfates: Generally soluble, except when paired with lead(II), barium, calcium, and strontium.
• Nitrates and Acetates: Usually soluble with no common exceptions.
• Halides (Chlorides, Bromides, Iodides): Often soluble, except for those involving silver, lead(II), and mercury(I).

For example, zinc chloride (ZnCl , is soluble because zinc is not one of the exceptions for halides. Similarly, strontium acetate, Sr(CH₃COO)₂, also dissolves well due to acetates' high solubility tendencies.

Alkaline Earth Metals

The alkaline earth metals are found in Group 2 of the periodic table and include elements such as beryllium, magnesium, calcium, strontium, barium, and radium. These metals are well-known for their unique properties and reactivity.

One significant property of interest is their behavior in water and their impact on the solubility of compounds:

• In general, sulfides and hydroxides of most alkaline earth metals like calcium, strontium, and barium are more soluble than those of magnesium and beryllium.
• Carbonates and phosphates of these metals tend to be insoluble.

For example, magnesium sulfide, MgS, exhibits solubility owing to magnesium's status as an alkaline earth metal, which is an exception to the typical insolubility of sulfides. Similarly, strontium forms soluble acetates, consistent with the general solubility of acetates.

These solubility patterns help predict how these metals will interact in various chemical contexts, making them fundamental for chemical intuition and practical lab work.