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) \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) (b) \(\mathrm{Al}(\mathrm{OH})_{3}\) (c) \(\mathrm{CuCO}_{3}\) (d) \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}\) (e) \(\mathrm{Ca}(\mathrm{ClO})_{2}\) (f) \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

(a) 1 \( \mathrm{Mg^{2+}} \), 2 \( \mathrm{CH_3CO_2^-} \); (b) 1 \( \mathrm{Al^{3+}} \), 3 \( \mathrm{OH^-} \); (c) 1 \( \mathrm{Cu^{2+}} \), 1 \( \mathrm{CO_3^{2-}} \); (d) 3 \( \mathrm{NH_4^+} \), 1 \( \mathrm{PO_4^{3-}} \); (e) 1 \( \mathrm{Ca^{2+}} \), 2 \( \mathrm{ClO^-} \); (f) 1 \( \mathrm{Na^+} \), 1 \( \mathrm{CH_3CO_2^-} \).

Step 1: Identify ions in Mg(CH3CO2)2

The compound \( \mathrm{Mg(CH_3CO_2)_2} \) consists of magnesium ion \( \mathrm{Mg^{2+}} \) and acetate ion \( \mathrm{CH_3CO_2^-} \). As there are two acetate ions per formula unit, the formula indicates one magnesium ion and two acetate ions per molecule.

Step 2: Identify ions in Al(OH)3

\( \mathrm{Al(OH)_3} \) is composed of aluminum ion \( \mathrm{Al^{3+}} \) and hydroxide ion \( \mathrm{OH^-} \). There is one aluminum ion and three hydroxide ions.

Step 3: Identify ions in CuCO3

In \( \mathrm{CuCO_3} \), copper(II) carbonate consists of copper ion \( \mathrm{Cu^{2+}} \) and carbonate ion \( \mathrm{CO_3^{2-}} \). The formula includes one copper ion and one carbonate ion.

Step 4: Identify ions in (NH4)3PO4

\( (\mathrm{NH_4})_3\mathrm{PO_4} \) combines ammonium ion \( \mathrm{NH_4^+} \) with phosphate ion \( \mathrm{PO_4^{3-}} \). The compound has three ammonium ions and one phosphate ion.

Step 5: Identify ions in Ca(ClO)2

\( \mathrm{Ca(ClO)_2} \) is composed of calcium ion \( \mathrm{Ca^{2+}} \) and hypochlorite ion \( \mathrm{ClO^-} \). For each molecule, there is one calcium ion and two hypochlorite ions.

Step 6: Identify ions in NaCH3CO2

The compound \( \mathrm{NaCH_3CO_2} \) is made of sodium ion \( \mathrm{Na^+} \) and acetate ion \( \mathrm{CH_3CO_2^-} \). It contains one sodium ion and one acetate ion.

Key Concepts

Chemical Formulas

In studying chemistry, the ability to interpret and write chemical formulas is fundamental. A chemical formula represents a compound's composition, showing the kinds and numbers of atoms or ions involved. For ionic compounds, these formulas reveal the ratios of ions.
Understanding these formulas helps in identifying how many atoms or ions of each type are present in a molecule, offering insights into the compound's structure and properties. Here's a breakdown of a few examples from the exercise:

• For the compound \(\mathrm{Mg(CH_3CO_2)_2}\), the formula indicates it’s made up of one magnesium ion (\(\mathrm{Mg^{2+}}\)) and two acetate ions (\(\mathrm{CH_3CO_2^-}\)).
• The compound \(\mathrm{CuCO_3}\) consists of one copper ion (\(\mathrm{Cu^{2+}}\)) and one carbonate ion (\(\mathrm{CO_3^{2-}}\)).

These formulas guide chemists in balancing chemical equations and predicting the results of chemical reactions.

Ion Identification

In the realm of ionic compounds, identifying the ions present is crucial for understanding the compound's nature and reactivity. Ions are atoms or molecules that have gained or lost electrons, resulting in a net electrical charge.
To identify ions in a compound's formula:

• Look for the individual elements or groups of elements that combine to form neutral compounds.
• Translate the compound formula to its ionic components, noting the type and number of each.

For example, in \(\mathrm{Al(OH)_3}\), the ions are aluminum (\(\mathrm{Al^{3+}}\)) and hydroxide (\(\mathrm{OH^-}\)), where one aluminum ion pairs with three hydroxide ions to maintain charge balance. Identifying ions correctly ensures accurate interpretation of the chemical's formula and properties.

Charges of Ions

Understanding the charges of ions is critical when dealing with ionic compounds, as these charges dictate how ions bond with each other. Ions can be positively charged (cations) or negatively charged (anions) depending on whether they lose or gain electrons.
Here’s how to understand these charges:

• Cations, like \(\mathrm{Mg^{2+}}\), result from an atom losing electrons, thus acquiring a positive charge.
• Anions, such as \(\mathrm{CH_3CO_2^-}\), gain electrons, acquiring a negative charge.

The net charge of any ionic compound must be zero, meaning the total positive charges must equal the total negative charges.
In \(\mathrm{(NH_4)_3PO_4}\), three ammonium ions (each \(\mathrm{NH_4^+}\)) exactly balance the charge with one phosphate ion (\(\mathrm{PO_4^{3-}}\)). Such knowledge of charges ensures the correct formation of stable compounds.

Polyatomic Ions

Polyatomic ions play a significant role in the chemistry of ionic compounds. They are ions that consist of two or more atoms covalently bonded, which collectively carry a charge.
They often appear in chemical formulas and are integral to understanding compound structure. Some common polyatomic ions include:

• Acetate \(\mathrm{CH_3CO_2^-}\), found in \(\mathrm{NaCH_3CO_2}\).
• Carbonate \(\mathrm{CO_3^{2-}}\), present in \(\mathrm{CuCO_3}\).
• Phosphate \(\mathrm{PO_4^{3-}}\), in \(\mathrm{(NH_4)_3PO_4}\).

Each of these polyatomic ions contains a specific number of atoms from different elements and possess a charge, which affects how they interact and form bonds with other ions. By understanding polyatomic ions, chemists can predict the behavior of ionic compounds under various conditions.