Question

Predict which of the following compounds are soluble in water: (a) aluminum nitrate, \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) (b) sodium sulfate, \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (c) cobalt(II) hydroxide, \(\mathrm{Co}(\mathrm{OH})_{2}\) (d) iron(III) phosphate, \(\mathrm{FePO}_{4}\)

Short answer

Aluminum nitrate and sodium sulfate are soluble in water; cobalt(II) hydroxide and iron(III) phosphate are not.

Step-by-step solution

Review Solubility Rules

To predict whether a compound is soluble in water, we must reference the general solubility rules. Most nitrates (NO₃⁻) and alkali metal salts (group 1 elements) are soluble. Hydroxides and phosphates are generally insoluble, with certain exceptions.

Analyze Aluminum Nitrate, \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\)

According to the solubility rules, all nitrates are soluble in water. Since aluminum nitrate contains the nitrate ion, it is soluble in water.

Analyze Sodium Sulfate, \(\mathrm{Na}_{2} \mathrm{SO}_{4}\)

Sodium is an alkali metal. Compounds with alkali metals are generally soluble in water. Therefore, sodium sulfate is soluble in water.

Analyze Cobalt(II) Hydroxide, \(\mathrm{Co}( OH)_{2}\)

Most hydroxides are insoluble in water, except for those of alkali metals and some alkaline earth metals like barium, strontium, and calcium to some extent. Since cobalt(II) hydroxide does not fall into these exceptions, it is insoluble in water.

Analyze Iron(III) Phosphate, \(\mathrm{FePO}_{4}\)

Phosphates are typically insoluble except when paired with alkali metals or ammonium. Since iron(III) phosphate does not contain any of these soluble exceptions, it is insoluble in water.

Key concepts

Nitrate Solubility

Nitrates are commonly noted for their high solubility in water. This rule is nearly universal for compounds containing the nitrate ion, \(\text{NO}_3^-\). The reason behind this is due to the dissociation of the nitrate ion in water, allowing the compound to dissolve readily.
When asked whether a nitrate compound, like aluminum nitrate, \(\text{Al}(\text{NO}_3)_3\), is soluble, the answer is a resounding yes. \( ext{NO}_3^-\) ions are very effective at breaking apart the structure of the solid compound, helping it dissolve. Thus, when you see a nitrate compound, you can almost always assume it is soluble.
For students encountering nitrate solubility rules in problems, remember:

• Nitrate-containing compounds are generally soluble in water.
• There are very few exceptions to nitrate solubility, making them reliable indicators of solubility.

Alkali Metal Solubility

Alkali metals encompass a group of elements found in Group 1 of the periodic table: lithium, sodium, potassium, rubidium, cesium, and francium. These metals exhibit a high degree of solubility in water across their compounds due to their single-valence electron configuration, which helps them dissociate easily in water.
Take sodium sulfate, \(\text{Na}_2\text{SO}_4\), as an example. Sodium, being an alkali metal, ensures that the compound is soluble. Alkali metal cations are incredibly adept at interacting with water molecules, promoting solubility.
Key points for remembering alkali metal solubility are:

• Compounds with alkali metals (Na, K, Li, etc.) are generally soluble in water.
• Alkali metals serve as excellent markers for predicting solubility in chemical compounds.

Hydroxide Solubility

Hydroxide ions, \(\text{OH}^-\), usually lead to insolubility in water, with exceptions mostly limited to hydroxides of alkali metals and some alkaline earth metals. Hydroxides like cobalt(II) hydroxide, \(\text{Co(OH)}_2\), are typically insoluble due to strong ionic bonds that water molecules often fail to break.
Exceptions in hydroxide solubility involve metals like barium, strontium, and calcium. These ions can form soluble hydroxides due to their large ionic sizes and corresponding weaker lattice energy.
For better understanding of hydroxide solubility, remember:

• The general rule is that most hydroxides are insoluble.
• Exceptions include alkali metal hydroxides (NaOH, KOH, etc.), and certain alkaline earth metal hydroxides (Ba(OH)_2, Sr(OH)_2).

Phosphate Solubility

Phosphates, which contain the \(\text{PO}_4^{3-}\) ion, are typically insoluble in water. This is largely due to the multiple negative charges and large ionic size of the phosphate ion. Such characteristics make it challenging for water molecules to separate the phosphate ion into an aqueous solution.
An example is iron(III) phosphate, \(\text{FePO}_4\), which is insoluble. Phosphate compounds tend to form sturdy ionic lattices, resistant to the dissociating action of water.
When learning about phosphate solubility, consider these points:

• Phosphate compounds are usually insoluble, with few exceptions.
• The exceptions often include compounds with alkali metals or ammonium ions, which can enhance solubility by weakening the ionic lattice.