Problem 44 - Chemistry and Chemical Reactivity

Question

Give the formula and the number of each ion that makes up each of the following compounds: (a) \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) (d) \(\mathrm{Ti}\left(\mathrm{SO}_{4}\right)_{2}\) (b) \(\mathrm{Al}(\mathrm{OH})_{3}\) (e) \(\mathrm{KH}_{2} \mathrm{PO}_{4}\) (c) \(\mathrm{CuCO}_{3}\) (f) \(\mathrm{CaHPO}_{4}\)

Step-by-Step Solution

Answer

The ions are: (a) \(\mathrm{Mg}^{2+}\) and \(2\, \mathrm{CH}_{3}\mathrm{CO}_{2}^{-}\), (d) \(\mathrm{Ti}^{4+}\) and \(2\, \mathrm{SO}_{4}^{2-}\), (b) \(\mathrm{Al}^{3+}\) and \(3\, \mathrm{OH}^{-}\), (e) \(\mathrm{K}^{+}\) and \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\), (c) \(\mathrm{Cu}^{2+}\) and \(\mathrm{CO}_{3}^{2-}\), (f) \(\mathrm{Ca}^{2+}\) and \(\mathrm{HPO}_{4}^{2-}\).

Step 1: Identify ions in \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\)

The compound \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) contains magnesium ions \(\mathrm{Mg}^{2+}\) and acetate ions \(\mathrm{CH}_{3}\mathrm{CO}_{2}^{-}\). Since there are two acetate ions for each magnesium ion, the ions are \(\mathrm{Mg}^{2+}\) and \(2\, \mathrm{CH}_{3}\mathrm{CO}_{2}^{-}\).

Step 2: Identify ions in \(\mathrm{Ti}\left(\mathrm{SO}_{4}\right)_{2}\)

The compound \(\mathrm{Ti}\left(\mathrm{SO}_{4}\right)_{2}\) contains titanium ions \(\mathrm{Ti}^{4+}\) and sulfate ions \(\mathrm{SO}_{4}^{2-}\). There are two sulfate ions for one titanium ion, so the ions are \(\mathrm{Ti}^{4+}\) and \(2\, \mathrm{SO}_{4}^{2-}\).

Step 3: Identify ions in \(\mathrm{Al}(\ ext{OH})_{3}\)

The compound \(\mathrm{Al}(\mathrm{OH})_{3}\) consists of aluminum ions \(\mathrm{Al}^{3+}\) and hydroxide ions \(\mathrm{OH}^{-}\). There are three hydroxide ions per aluminum ion, so the ions are \(\mathrm{Al}^{3+}\) and \(3\, \mathrm{OH}^{-}\).

Step 4: Identify ions in \(\mathrm{KH}_{2} \mathrm{PO}_{4}\)

The compound \(\mathrm{KH}_{2} \mathrm{PO}_{4}\) consists of potassium ions \(\mathrm{K}^{+}\) and dihydrogen phosphate ions \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\). There is one ion of each type per formula unit, so the ions are \(\mathrm{K}^{+}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\).

Step 5: Identify ions in \(\mathrm{CuCO}_{3}\)

The compound \(\mathrm{CuCO}_{3}\) contains copper ions \(\mathrm{Cu}^{2+}\) and carbonate ions \(\mathrm{CO}_{3}^{2-}\). There is one of each ion per formula unit, so the ions are \(\mathrm{Cu}^{2+}\) and \(\mathrm{CO}_{3}^{2-}\).

Step 6: Identify ions in \(\mathrm{CaHPO}_{4}\)

The compound \(\mathrm{CaHPO}_{4}\) consists of calcium ions \(\mathrm{Ca}^{2+}\) and hydrogen phosphate ions \(\mathrm{HPO}_{4}^{2-}\). There is one ion of each per formula unit, so the ions are \(\mathrm{Ca}^{2+}\) and \(\mathrm{HPO}_{4}^{2-}\).

Key Concepts

Ion Identification

Identifying ions in a chemical compound is a foundational step in understanding the formula composition. This process involves looking at the components of a compound to determine which ions are present. For example, the compound \( \mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \) includes magnesium ions \( \mathrm{Mg}^{2+} \) and acetate ions \( \mathrm{CH}_{3}\mathrm{CO}_{2}^{-} \). Here, we see that for every magnesium ion, there are two acetate ions.

• **Magnesium Ion (Mg):** It is a metal ion, commonly found as \( \mathrm{Mg}^{2+} \) in compounds.
• **Acetate Ion (CH3CO2):** It is an organic ion with a single negative charge, \( \mathrm{CH}_{3}\mathrm{CO}_{2}^{-} \).

When identifying ions, it's critical to recognize common polyatomic ions as well as simple elemental ions. Each ion contributes to the overall charge balance in the compound.

Chemical Formulas

The chemical formula of a compound tells you the types and numbers of atoms present. For ionic compounds, it also gives clues about the ions involved. Taking \( \mathrm{Ti}\left(\mathrm{SO}_{4}\right)_{2} \) as an example, this formula suggests a titanium ion and two sulfate ions come together to form this compound. To read chemical formulas accurately, you must know:- **What each element symbol stands for**, like \( \mathrm{Ti} \) for titanium.- **Subscript numbers signify the quantity of each ion or group of atoms**; here, the subscript 2 outside the parentheses indicates there are two sulfate ions.Chemical formulas deploy these notations to define molecular composition precisely and succinctly, ensuring that the compound's neutrality in charge is evident.

Ion Charges

Ion charges are key to how ionic compounds balance themselves. An ion can have either a positive charge (cation) or a negative charge (anion), and these charges help in forming a stable compound. For instance, in \( \mathrm{Al}(\mathrm{OH})_{3} \), aluminum has a charge of \( \mathrm{Al}^{3+} \), and hydroxide ions have a charge of \( \mathrm{OH}^{-} \). Thus, three hydroxide ions align with one aluminum ion to balance the charges.

• **Cations (Positive Ions):** These often arise from metal atoms losing electrons, like \( \mathrm{Al}^{3+} \).
• **Anions (Negative Ions):** These result from non-metals or polyatomic groups gaining electrons, like \( \mathrm{OH}^{-} \).

Recognizing and balancing these charges are crucial in forming and predicting the correct chemical formulas.

Molecular Composition

Understanding molecular composition involves knowing the exact makeup of a molecule in terms of its atoms and ions. This can involve simple molecules or more complex structures, as seen with compounds like \( \mathrm{KH}_{2} \mathrm{PO}_{4} \). In this compound, there is one potassium ion \( \mathrm{K}^{+} \) and one dihydrogen phosphate ion \( \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \).1. **Identify all components**: Ensure all molecules or ions are named and accounted for in the chemical formula.2. **Count and balance**: Even though you have different ions, they balance their charges as part of the complete structure, preserving overall neutrality.Molecular composition provides vital insight into how compounds react, dissolve, or engage with other molecules, impacting everything from chemical reactions to biological processes.