Problem 82 - Introductory Chemistry Atoms First

Question

Indicate whether each compound is soluble or insoluble in water: (a) \(\mathrm{BaCO}_{3}\) (b) \(\mathrm{K}_{2} \mathrm{~S}\) (c) \(\mathrm{CaSO}_{4}\) (d) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (e) \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}\)

Step-by-Step Solution

Answer

(a) \(BaCO_3\) is insoluble in water. (b) \(K_2S\) is soluble in water. (c) \(CaSO_4\) is insoluble in water. (d) \(Mg(NO_3)_2\) is soluble in water. (e) \((NH_4)_3PO_4\) is soluble in water.

Step 1: (a) BaCO₃ solubility

According to solubility rule 6, most carbonate salts are insoluble, except those of Group 1 metals and ammonium. Since Ba²⁺ is not a Group 1 metal or ammonium, BaCO₃ is considered insoluble in water.

Step 2: (b) K₂S solubility

According to solubility rule 2, all salts of alkali metals (Group 1) are soluble. Since potassium (K) is an alkali metal, K₂S is soluble in water.

Step 3: (c) CaSO₄ solubility

According to solubility rule 4, most sulfate salts are soluble, except those of barium, calcium, silver, lead, and strontium. Since Ca²⁺ is one of the exceptions, CaSO₄ is considered insoluble in water.

Step 4: (d) Mg(NO₃)₂ solubility

According to solubility rule 1, all nitrate salts are soluble. Since Mg(NO₃)₂ is a nitrate salt, it is soluble in water.

Step 5: (e) (NH₄)₃PO₄ solubility

According to solubility rule 6, most phosphate salts are insoluble, except those of Group 1 metals and ammonium. Since (NH₄)₃PO₄ contains ammonium, it is considered soluble in water.

Key Concepts

Carbonate Salts

Carbonate salts are compounds made up of carbonate ions \(\text{CO}_3^{2-}\) and metal cations. Generally, these salts tend to be insoluble in water. The reason stems from the lattice energy of the carbonate compound being greater than the energy released when they dissolve. However, there are exceptions to the rule: carbonate salts formed with alkali metals (Group 1 elements) and ammonium \(\text{NH}_4^+\) are soluble.

For example, in the exercise, \(\text{BaCO}_3\) (barium carbonate) is found to be insoluble since barium isn't a Group 1 element or ammonium. Remember:

• Most carbonate salts are insoluble.
• Alkali metal and ammonium carbonates are exceptions.

No need to memorize every rule, just remember the exceptions to carbonate's insolubility are key.

Sulfate Salts

Sulfate salts contain sulfate ions \(\text{SO}_4^{2-}\) paired with various metal cations. A general rule of thumb is that most sulfate salts dissolve in water, making them soluble. However, there are notable exceptions, including those with barium, lead, calcium, and strontium.

In the exercise, \(\text{CaSO}_4\) (calcium sulfate) is one of these exceptions, making it insoluble. When you come across sulfates, remember:

• Most sulfate salts are soluble.
• Exceptions include barium, calcium, lead, and strontium sulfates.

This should help clarify which sulfates dissolve and which do not.

Nitrate Salts

Nitrate salts are compounds that pair metal cations with nitrate ions \(\text{NO}_3^{-}\). They hold the unique distinction of being universally soluble in water. This rule is simple, easy to remember, and widely applicable. Hence, any salt containing the nitrate ion will dissolve, making these compounds extremely predictable.

As seen in the exercise, \(\text{Mg}(\text{NO}_3)_2\) (magnesium nitrate) follows this rule, being soluble in water. Key points to remember:

• All nitrate salts are soluble, no exceptions.

This simplicity makes nitrate salts a reliable aspect of solubility rules.

Phosphate Salts

Phosphate salts consist of phosphate ions \(\text{PO}_4^{3-}\) joined with metal cations. Generally, these salts are insoluble in water, but, much like carbonate salts, there are exceptions. Phosphate salts compound with alkali metals (Group 1) and ammonium \(\text{NH}_4^+\) do dissolve.

In the exercise, \(\text{(NH}_4\text{)}_3\text{PO}_4\) (ammonium phosphate) is soluble because it contains the ammonium ion. For phosphates, remember:

• Most phosphate salts are insoluble.
• Exceptions are alkali metal and ammonium phosphates.

Understanding these exceptions can help demystify phosphate solubility.

Alkali Metals

Alkali metals belong to Group 1 in the periodic table, highlighting elements like lithium, sodium, potassium, and others. Compounds formed with alkali metals are pervasive in chemistry due to their highly reactive nature. One defining characteristic is the universal solubility of their salts in water, whether it's with carbonates, sulfates, nitrates, or phosphates.

In the exercise, \(\text{K}_2\text{S}\) (potassium sulfide) exemplifies this rule's application, being soluble due to potassium's inclusion as an alkali metal. Key takeaways about alkali metals:

• Alkali metal salts are almost always soluble, regardless of the anion paired with them.

This knowledge serves as a cornerstone for understanding broader solubility rules.