Question

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

Short answer

(a) Insoluble, (b) Insoluble, (c) Soluble, (d) Insoluble, (e) Soluble.

Step-by-step solution

Solubility Rules Overview

Understand the basic solubility rules for ionic compounds in water. For example, nitrates (NO₃⁻) and acetates (CH₃COO⁻) are generally soluble in water. Chlorides (Cl⁻) are mostly soluble, except with Ag⁺, Pb²⁺, and Hg₂²⁺. Sulfides (S²⁻) are generally insoluble, except with alkali metals and alkaline earth metals like Mg. Hydroxides (OH⁻) are mostly insoluble except with alkali metals and Ca²⁺, Sr²⁺, Ba²⁺.

Evaluate Solubility of MgS

Check if MgS is soluble. Sulfides are generally insoluble unless paired with alkali metals or Mg, Ca, Sr, or Ba. In this case, Mg is one of the exceptions for sulfide solubility, making MgS insoluble in water.

Evaluate Solubility of Cr(OH)₃

Consider the solubility rules for hydroxides. Hydroxides are generally insoluble except for alkali metals and some alkaline earth metals like Ba. Chromium(III) hydroxide, Cr(OH)₃, does not fit the exceptions and is therefore insoluble in water.

Evaluate Solubility of ZnCl₂

Chlorides are typically soluble in water, with exceptions for Ag⁺, Pb²⁺, and Hg₂²⁺ salts. ZnCl₂ does not contain any of these exceptions, making it soluble in water.

Evaluate Solubility of Pb₃(PO₄)₂

Examine the solubility of phosphates. Phosphates are generally insoluble in water except when combined with alkali metals. Pb₃(PO₄)₂, having lead ions, does not fall under any exceptions and is insoluble in water.

Evaluate Solubility of Sr(CH₃COO)₂

Analyze the solubility rules for acetates. Acetates are generally soluble in water, with no known common exceptions. Thus, Sr(CH₃COO)₂ is soluble in water.

Key concepts

Ionic Compounds

Ionic compounds are made up of positively charged ions, known as cations, and negatively charged ions, known as anions. When these oppositely charged ions bind together, they form a neutral compound. Due to their defined structure, ionic compounds have unique properties such as high melting and boiling points, and they conduct electricity when dissolved in water. This conductivity stems from the free-moving ions in solution.
Ionic compounds typically dissolve in water because of the polar nature of water molecules. Water molecules surround and separate the individual ions, breaking the ionic bond. However, this solubility can vary significantly depending on various ions' properties and is often governed by specific solubility rules.

Solubility of Sulfides

Sulfides are compounds composed of the sulfide ion, \((\mathrm{S}^{2-})\), paired with different cations. Generally, sulfides are insoluble in water, making them less likely to dissolve. However, there are exceptions to this rule. Sulfides that are paired with alkali metal cations such as sodium (Na⁺), potassium (K⁺), and some alkaline earth metals like magnesium (Mg⁺), calcium (Ca⁺), strontium (Sr⁺), and barium (Ba⁺) can be soluble.
When dealing with magnesium sulfide \(\mathrm{MgS}\), it initially appears insoluble, but magnesium is one of the cations known to form soluble sulfide compounds. Despite this, this compound's specific solubility might still vary depending on conditions and concentrations.

Solubility of Hydroxides

Hydroxides contain the hydroxide ion \(\mathrm{(OH)^{-}}\), and their solubility varies widely across different compounds. Most hydroxides are insoluble in water. The notable exceptions include hydroxides of alkali metals like sodium \(\mathrm{(NaOH)}\) and potassium \(\mathrm{(KOH)}\), which are soluble. Hydroxides of some alkaline earth metals are also exceptions, specifically calcium \(\mathrm{(Ca(OH)_2)}\), and strontium \(\mathrm{(Sr(OH)_2)}\), and barium \(\mathrm{(Ba(OH)_2)}\).
When considering chromium(III) hydroxide, \(\mathrm{Cr(OH)_3}\), it does not align with these soluble exceptions, resulting in its classification as an insoluble compound. As with many insoluble ionic compounds, this does not dissolve appreciably in water.

Solubility of Chlorides

Chlorides are compounds that contain the chloride ion \(\mathrm{(Cl^{-})}\). Most chlorides are soluble in water. However, there are notable exceptions such as those containing silver \(\mathrm{Ag^+}\), lead \(\mathrm{Pb^{2+}}\), and mercury \(\mathrm{(Hg_2^{2+})}\) ions, which render those particular chlorides insoluble.
Zinc chloride \(\mathrm{(ZnCl_2)}\) does not fall under these exceptions and thus is water-soluble. This means when zinc chloride is in contact with water, it dissociates into its component ions - zinc ions \(\mathrm{(Zn^{2+})}\) and chloride ions - allowing it to integrate into the solution efficiently.

Solubility of Phosphates

Phosphates, containing the phosphate ion \(\mathrm{(PO_{4}^{3-})}\), generally have low solubility in water. These compounds are often insoluble except when paired with alkali metals like sodium, lithium, and potassium, wherein they become soluble. The insolubility arises due to the strong ionic bonds within phosphates and their large polyatomic form.
For example, lead(II) phosphate \(\mathrm{(Pb_{3}(PO_{4})_{2})}\) does not fall under the category of alkali metal phosphates and therefore remains insoluble in water. Such phosphates are often used in industrial applications where minimal solubility is beneficial.

Solubility of Acetates

Acetates, which contain the acetate ion \(\mathrm{(CH_{3}COO^{-})}\), are typically soluble in water, across a wide range of counterparts. This universal solubility is due to the acetate ion's unique structure and interaction with water molecules, which stabilize it in solution. No well-known exceptions exist, making acetates one of the most reliably soluble categories of compounds.
Strontium acetate \(\mathrm{(Sr(CH_{3}COO)_{2})}\) follows the general rule and is, therefore, soluble in water. When dissolved, it disassociates into its ions and integrates smoothly into the aqueous environment, highlighted by its availability for chemical reactions or applications that require dissolved acetates.