Question

Specify what ions are present upon dissolving each of the following substances in water: \((\mathbf{a}) \mathrm{HIO}_{3},(\mathbf{b}) \mathrm{Ba}(\mathrm{OH})_{2},\) (c) HCN, (d) \(\mathrm{CuSO}_{4}\).

Short answer

(a) H⁺ and IO₃⁻; (b) Ba²⁺ and OH⁻; (c) H⁺ and CN⁻; (d) Cu²⁺ and SO₄²⁻.

Step-by-step solution

Analyze the Dissolution of HIO3

HIO₃, iodic acid, is a strong acid that completely dissociates in water. When dissolved in water, it dissociates into hydrogen ions (H⁺) and iodate ions (IO₃⁻). Thus, the ions present in the solution are H⁺ and IO₃⁻.

Analyze the Dissolution of Ba(OH)2

Barium hydroxide, Ba(OH)₂, is a strong base and dissociates completely in water. It dissociates into barium ions (Ba²⁺) and hydroxide ions (OH⁻). For each formula unit of Ba(OH)₂, one Ba²⁺ ion and two OH⁻ ions are produced in the solution.

Analyze the Dissolution of HCN

Hydrogen cyanide, HCN, is a weak acid and only partially dissociates in water. When it dissociates, it forms hydrogen ions (H⁺) and cyanide ions (CN⁻). However, because it is a weak acid, not all HCN molecules will dissociate; only some will produce ions in the solution.

Analyze the Dissolution of CuSO4

Copper(II) sulfate, CuSO₄, when dissolved in water, dissociates into copper(II) ions (Cu²⁺) and sulfate ions (SO₄²⁻). These ions are present in the solution in equal molar amounts, as each formula unit of CuSO₄ dissociates into one Cu²⁺ ion and one SO₄²⁻ ion.

Key concepts

Strong Acids and Bases

When we talk about strong acids and bases, we're referring to substances that undergo complete dissociation in water.
This means that when a strong acid or a strong base dissolves in water, it completely breaks apart into ions.

• Strong Acids: These include common acids like hydrochloric acid (HCl) and sulfuric acid (H₂SO₄). As in the example of iodic acid (HIO₃), when dissolved, it splits into hydrogen ions (H⁺) and iodate ions (IO₃⁻). There's no HIO₃ left in its molecular form once it's dissolved.


• Strong Bases: Strong bases, such as barium hydroxide (Ba(OH)₂), also dissociate completely. Ba(OH)₂ in water separates into barium ions (Ba²⁺) and hydroxide ions (OH⁻). Importantly, each mole of Ba(OH)₂ produces two moles of OH⁻ ions, boosting the pH of the solution significantly.

Understanding complete dissociation helps predict the behavior of solutions made from strong acids and bases, particularly their conductive properties and pH levels.

Weak Acids

Weak acids are a bit different because they only partially dissociate in water.
This means if you dissolve a weak acid like hydrogen cyanide (HCN) in water, only a small portion of it breaks down into ions.
Most of the weak acid remains in its original molecular form.

• For HCN, only some molecules produce hydrogen ions (H⁺) and cyanide ions (CN⁻), which means the solution contains both these ions and non-dissociated HCN molecules.
• Because they don't fully dissociate, weak acids contribute to a much lower concentration of free hydrogen ions in the solution, leading to a higher pH compared to strong acids.

This partial ionization is an important concept because it affects the acidity and conductive properties of the solution.

Ionization in Water

In chemistry, ionization in water refers to how substances dissolve and break apart into ions when mixed with water.
This process plays a crucial role because the presence of ions in water allows the solution to conduct electricity.

• When a substance like copper(II) sulfate (CuSO₄) dissolves, it ionizes into copper ions (Cu²⁺) and sulfate ions (SO₄²⁻).
• The extent of ionization varies; strong acids and bases ionize completely, while weak acids and bases ionize partially.

Understanding ionization helps in predicting the behavior of the solution in terms of its electrical conductivity, reactivity, and acidity or basicity (pH levels). The dissociation into ions affects how compounds interact in aqueous environments, crucial for many biological and chemical processes.

Chemical Nomenclature

Chemical nomenclature is the system used to name chemical compounds. Understanding it is key to decoding the formulas and naming the compounds correctly.

• Acids: In acids like HIO₃ (iodic acid) and HCN (hydrogen cyanide), the nomenclature often gives away the types of ions produced when these acids dissociate.
• Bases: Names like barium hydroxide indicate that a hydroxide ion (OH⁻) is released upon dissociation.
• Salts: The name copper(II) sulfate reveals a sulfate ion (SO₄²⁻) and indicates the copper ion’s charge, Cu²⁺.

By familiarizing yourself with chemical nomenclature, you'll have a helpful tool for understanding and predicting the compounds' behaviors, and you'll be able to communicate more effectively in scientific contexts.