Problem 18 - Chemistry The Central Science

Question

Specify what ions are present upon dissolving each of the following substances in water: (a) \(\mathrm{MgI}_{2,}\) (b) \(\mathrm{K}_{2} \mathrm{CO}_{3,}$$(\mathbf{c})$$\mathrm{HClO}_{4,}\) \((\mathbf{d}) \mathrm{NaCH}_{3} \mathrm{COO.}\)

Step-by-Step Solution

Answer

Upon dissolving the given substances in water, the following ions are produced: (a) \(\mathrm{MgI}_2\): \(\mathrm{Mg}^{2+}(aq)\) and \(2\mathrm{I}^{-}(aq)\) (b) \(\mathrm{K}_2\mathrm{CO}_3\): \(2\mathrm{K}^{+}(aq)\) and \(\mathrm{CO}_3^{2-}(aq)\) (c) \(\mathrm{HClO}_4\): \(\mathrm{H}^{+}(aq)\) and \(\mathrm{ClO}_4^{-}(aq)\) (d) \(\mathrm{NaCH}_3\mathrm{COO}\): \(\mathrm{Na}^{+}(aq)\) and \(\mathrm{CH}_3\mathrm{COO}^{-}(aq)\)

Step 1: (a) Magnesium iodide (\(\mathrm{MgI}_2\))

When magnesium iodide (\(\mathrm{MgI}_2\)) is dissolved in water, it dissociates into its component ions: one magnesium ion and two iodide ions. These ions are represented as: $$ \mathrm{MgI}_{2} \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{I}^{-}(aq) $$

Step 2: (b) Potassium carbonate (\(\mathrm{K}_2 \mathrm{CO}_3\))

When potassium carbonate (\(\mathrm{K}_2 \mathrm{CO}_3\)) is dissolved in water, it dissociates into its component ions: two potassium ions and one carbonate ion. These ions are represented as: $$ \mathrm{K}_{2} \mathrm{CO}_{3} \rightarrow 2 \mathrm{K}^{+}(aq) + \mathrm{CO}_3^{2-}(aq) $$

Step 3: (c) Perchloric acid (\(\mathrm{HClO}_4\))

When perchloric acid (\(\mathrm{HClO}_4\)) is dissolved in water, it dissociates into its component ions: one hydrogen ion (proton) and one perchlorate ion. These ions are represented as: $$ \mathrm{HClO}_4 \rightarrow \mathrm{H}^{+}(aq) + \mathrm{ClO}_4^{-}(aq) $$

Step 4: (d) Sodium acetate (\(\mathrm{NaCH}_3 \mathrm{COO}\))

When sodium acetate (\(\mathrm{NaCH}_3 \mathrm{COO}\)) is dissolved in water, it dissociates into its component ions: one sodium ion and one acetate ion. These ions are represented as: $$ \mathrm{NaCH}_3 \mathrm{COO} \rightarrow \mathrm{Na}^{+}(aq) + \mathrm{CH}_3 \mathrm{COO}^{-}(aq) $$ To summarize, the ions produced when the given substances are dissolved in water are: (a) \(\mathrm{MgI}_2\): \(\mathrm{Mg}^{2+}(aq)\) and \(2\mathrm{I}^{-}(aq)\) (b) \(\mathrm{K}_2\mathrm{CO}_3\): \(2\mathrm{K}^{+}(aq)\) and \(\mathrm{CO}_3^{2-}(aq)\) (c) \(\mathrm{HClO}_4\): \(\mathrm{H}^{+}(aq)\) and \(\mathrm{ClO}_4^{-}(aq)\) (d) \(\mathrm{NaCH}_3\mathrm{COO}\): \(\mathrm{Na}^{+}(aq)\) and \(\mathrm{CH}_3\mathrm{COO}^{-}(aq)\)

Key Concepts

Dissociation of Compounds

When ionic compounds are dissolved in water, they break apart or dissociate into their respective ions. This process is essential for conducting electricity in solutions. For example, when table salt (NaCl) is dissolved in water, it dissociates into sodium ions and chloride ions. The equation for dissociation is written as:

• NaCl(s) → Na⁺(aq) + Cl^-(aq)

In the solid state, these ions are bound tightly in a lattice structure. Yet, when in water, they separate and become surrounded by water molecules, resulting in freely moving ions (aqueous ions). This capability of dissociation is what makes ionic solutions conductive.

Magnesium Iodide

Magnesium iodide, with the chemical formula \(\mathrm{MgI}_2\), consists of magnesium ions (Mg²⁺) and iodide ions (I^-). When it dissolves in water, it dissociates as follows:

• \(\mathrm{MgI}_{2} \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{I}^{-}(aq)\)

Here, each formula unit of magnesium iodide yields one magnesium ion and two iodide ions. The magnesium ion carries a +2 charge, while each iodide ion carries a -1 charge. Magnesium iodide is highly soluble in water, which means these ions easily separate and remain in solution.

Potassium Carbonate

Potassium carbonate, \(\mathrm{K}_2 \mathrm{CO}_3\), dissolves in water by dissociating into potassium ions (K⁺) and carbonate ions (CO₃^2-). The dissociation equation is:

• \(\mathrm{K}_{2} \mathrm{CO}_{3} \rightarrow 2 \mathrm{K}^{+}(aq) + \mathrm{CO}_3^{2-}(aq)\)

This indicates that each unit of potassium carbonate produces two potassium ions and one carbonate ion in aqueous solution. Potassium ions are positively charged with a single positive charge, and carbonate ions are negatively charged with two negative charges. Potassium carbonate is commonly used in the production of glass and soap.

Perchloric Acid

Perchloric acid is a strong acid denoted by the formula \(\mathrm{HClO}_4\). It fully dissociates in water into hydrogen ions (H⁺) and perchlorate ions (ClO₄^-):

• \(\mathrm{HClO}_4 \rightarrow \mathrm{H}^{+}(aq) + \mathrm{ClO}_4^{-}(aq)\)

This complete dissociation signifies that perchloric acid releases one hydrogen ion for every molecule dissolved. The perchlorate ion has a single negative charge. Perchloric acid is among the strongest acids and is used in the laboratory setting as a precursor for various compounds and reactions.

Sodium Acetate

Sodium acetate, with the formula \(\mathrm{NaCH}_3 \mathrm{COO}\), breaks down in water to form sodium ions (Na⁺) and acetate ions (CH₃COO^-):

• \(\mathrm{NaCH}_3 \mathrm{COO} \rightarrow \mathrm{Na}^{+}(aq) + \mathrm{CH}_3 \mathrm{COO}^{-}(aq)\)

In solution, each sodium acetate unit yields one sodium ion, positive, and one acetate ion, negative. Sodium acetate is often used in the food industry as a preservative and flavoring agent. Additionally, it’s frequently utilized in heating pads as its crystallization from the liquid phase releases heat.

Aqueous Ions

Aqueous ions are ions that are free to move in a solution, typically water, that results from the dissociation of ionic compounds. When compounds like \(\mathrm{NaCl}\), \(\mathrm{MgI}_2\), or acids like \(\mathrm{HClO}_4\) are dissolved, their ions dissociate and are surrounded by water molecules.

• These ions ensure electrical conductivity in an aqueous solution.
• They play key roles in chemical reactions that occur in solutions.

Aqueous ions are essential for the transport of electrical signals in biological systems and are used in countless applications, from medicine to industrial processes, due to their reactive properties.