Problem 50 - Chemistry and Chemical Reactivity

Question

Name each of the following ionic compounds: (a) \(\mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) (b) \(\mathrm{Ni}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) \(\mathrm{KH}_{2} \mathrm{PO}_{4}\)

Step-by-Step Solution

Answer

(a) Calcium acetate, (b) Nickel(II) phosphate, (c) Aluminum hydroxide, (d) Potassium dihydrogen phosphate.

Step 1: Identify Ions in Compound (a)

The compound \( \mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \) consists of calcium ions \( \mathrm{Ca}^{2+} \) and acetate ions \( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \). The calcium ion is a metal cation, and acetate is a common polyatomic anion.

Step 2: Name Compound (a)

The name of the compound \( \mathrm{Ca}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \) is "calcium acetate," combining the metal cation name with the polyatomic anion.

Step 3: Identify Ions in Compound (b)

\( \mathrm{Ni}_{3}\left(\mathrm{PO}_{4}\right)_{2} \) includes nickel ions and phosphate ions. Nickel can form different charges, but in this compound, it would form \( \mathrm{Ni}^{2+} \) to balance the \( \mathrm{PO}_{4}^{3-} \) charge.

Step 4: Name Compound (b)

The compound \( \mathrm{Ni}_{3}\left(\mathrm{PO}_{4}\right)_{2} \) is named "nickel(II) phosphate," indicating the \( \mathrm{Ni}^{2+} \) state and the phosphate group.

Step 5: Identify Ions in Compound (c)

In \( \mathrm{Al}(\mathrm{OH})_{3} \), there are aluminum ions \( \mathrm{Al}^{3+} \) and hydroxide ions \( \mathrm{OH}^{-} \). Aluminum is a trivalent metal cation, and hydroxide is a monovalent anion.

Step 6: Name Compound (c)

The name of \( \mathrm{Al}(\mathrm{OH})_{3} \) is "aluminum hydroxide," which combines the cation name with the hydroxide group.

Step 7: Identify Ions in Compound (d)

\( \mathrm{KH}_{2} \mathrm{PO}_{4} \) consists of potassium ions \( \mathrm{K}^{+} \) and dihydrogen phosphate \( \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \). Potassium is typically a monovalent cation.

Step 8: Name Compound (d)

The compound \( \mathrm{KH}_{2} \mathrm{PO}_{4} \) is named "potassium dihydrogen phosphate." This reflects the potassium ion combined with the dihydrogen phosphate group.

Key Concepts

Calcium Acetate

Calcium acetate, represented by the chemical formula \( \mathrm{Ca} \left( \mathrm{CH}_{3} \mathrm{CO}_{2} \right)_{2} \), is an ionic compound made up of calcium ions (\( \mathrm{Ca}^{2+} \)) and acetate ions (\( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \)).
The name of the compound tells us its composition by combining the name of the metal cation, calcium, with the polyatomic anion, acetate. Acetate is a common anion derived from acetic acid.

**How to Write the Name:**

• Identify the cation: Calcium \( \mathrm{Ca}^{2+} \).
• Identify the polyatomic anion: Acetate \( \mathrm{CH}_{3} \mathrm{CO}_{2}^{-} \).
• Combine them: Name the cation followed by the anion, resulting in calcium acetate.

Calcium acetate is often used in various applications, such as a dietary supplement and in the food industry as a stabilizer.

Nickel(II) Phosphate

Nickel(II) phosphate is a vital compound in chemistry, represented by \( \mathrm{Ni}_{3} \left( \mathrm{PO}_{4} \right)_{2} \).
This compound includes three nickel ions with a \( \mathrm{Ni}^{2+} \) charge and two phosphate ions \( \mathrm{PO}_{4}^{3-} \). Nickel here specifically carries a \( \mathrm{Ni}^{2+} \) charge to balance the negative charge of the phosphate ions.

**Recognizing the Nickel Charge:**

• Nickel can have multiple ionic charges, typically +2 or +3.
• The "(II)" in nickel(II) indicates a divalent nickel ion \( \mathrm{Ni}^{2+} \).
• This notation helps distinguish it from any other potential charge states nickel may possess.

The systematic naming helps avoid confusion when dealing with transition metals like nickel, which commonly have variable oxidation states. Nickel(II) phosphate is used across various industries, including catalysts and battery materials.

Aluminum Hydroxide

Aluminum hydroxide is another essential ionic compound featuring the combination of aluminum and hydroxide ions.
It is denoted by \( \mathrm{Al} ( \mathrm{OH} )_{3} \), where the aluminum ion (\( \mathrm{Al}^{3+} \)) pairs with three hydroxide ions (\( \mathrm{OH}^{-} \)).

**Explaining the Name Aluminum Hydroxide:**

• Aluminum is a trivalent (3+) metal cation needing three \( \mathrm{OH}^{-} \) ions to balance the overall charge.
• Hydroxide is a simple polyatomic ion with a charge of -1.
• Combine them to form the compound's name: Aluminum hydroxide.

Aluminum hydroxide serves numerous functions, especially in the medical field, where it's used as an antacid to neutralize stomach acid.

Potassium Dihydrogen Phosphate

Potassium dihydrogen phosphate is represented by the formula \( \mathrm{KH}_{2} \mathrm{PO}_{4} \).
This compound consists of a potassium ion (\( \mathrm{K}^{+} \)) and a dihydrogen phosphate ion (\( \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \)). In this case, potassium is a monovalent cation, commonly encountered in various compounds.

**Naming Strategy for Potassium Dihydrogen Phosphate:**

• Identify the cation: Potassium \( \mathrm{K}^{+} \).
• Identify the anion: Dihydrogen phosphate \( \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \).
• Combine them by naming the cation first followed by the anion: Potassium dihydrogen phosphate.

This compound is especially valuable as a fertilizer and in laboratory settings as a chemical reagent.