Naming Ionic Hydrates: Videos & Practice Problems

Ionic hydrates are ionic compounds that incorporate water molecules into their structure, denoted by a dot separating the compound and the water molecules (e.g., CuX·H₂O). Naming ionic hydrates involves the same initial steps as naming regular ionic compounds, with the addition of a numerical prefix to indicate the number of water molecules present. These prefixes range from 'mono-' for one, up to 'deca-' for ten. The term 'hydrate' is used to describe the water portion. Understanding the correct naming convention for ionic hydrates is essential for accurately communicating their composition and distinguishing between different hydrates.

Ionic Hydrates Naming

Ionic hydrates are a specific type of ionic compound that are associated with at least one molecule of water. For example, in the formula \( \text{Cu}_x\text{H}_2\text{O} \), the variable \( x \) represents an unknown number of water molecules that are bonded to the copper compound. The dot in the formula indicates the connection between the ionic compound and the water molecules.

To name ionic hydrates, we follow a set of rules that are similar to those used for naming ionic compounds. The first three steps in the naming process mirror the rules for ionic compounds. If you need a refresher on these rules, it is beneficial to review the relevant materials on naming ionic compounds.

The fourth step introduces a new concept: the water molecules in the hydrate are referred to as "hydrates." To specify the number of water molecules present, we use numerical prefixes. These prefixes are as follows:

• Mono- for 1
• Di- for 2
• Tri- for 3
• Tetra- for 4
• Penta- for 5
• Hexa- for 6
• Hepta- for 7
• Octa- for 8
• Nona- for 9
• Deca- for 10

With this understanding of ionic hydrates and the rules for naming them, you can now apply this knowledge to practice problems and further solidify your grasp of the topic.

Ionic Hydrate Naming Example

To name the ionic hydrate in question, we first identify the number of water molecules present. In this case, there are five water molecules, which corresponds to the prefix "penta." Therefore, we refer to the water component as "pentahydrate."

Next, we analyze the ionic compound portion, which consists of lead and oxygen. Lead can exhibit two possible oxidation states: +2 or +4. Although lead is not classified as a transition metal, it shares this characteristic with tin. For oxygen, which belongs to group 6A of the periodic table, the typical charge is -2. Since there are two oxygen atoms in the formula, their combined charge is -4.

To achieve electrical neutrality in the compound PbO₂, the total negative charge from the oxygen must be balanced by a corresponding positive charge from lead. Thus, the +4 charge from lead cancels out the -4 charge from the two oxygen atoms, resulting in a neutral ionic compound.

Consequently, we denote lead with a +4 oxidation state using Roman numerals, leading to "lead(IV)." The base name for oxygen is "ox," and when we add the suffix "-ide," it becomes "oxide." Therefore, the complete name of the ionic compound is "lead(IV) oxide."

Combining both parts, the final name of the ionic hydrate is "lead(IV) oxide pentahydrate."

Give the systematic name from the given formula:K₂Cr₂O₇ ∙ 3 H₂O

• A. dipotassium dichromate trihydrate
• B. potassium dichromate trihydrate
• C. potassium chromate trihydrate
• D. trihydrate potassium chromate

Answer: (B)

Write the chemical formula for the following compound:Calcium selenide hexahydrate

• A. CaSe • 6 H₂O
• B. CaSe₂ • 6 H₂O
• C. Ca₂Se • 6 H₂O
• D. Ca₂Se₂ • 6 H₂O
• E. Ca₃Se₂ • 6 H₂O

Answer: (A)

Here’s what students ask on this topic:

How do you name hydrates?

Naming hydrates follows a specific nomenclature in chemistry. First, you name the ionic compound as you normally would. Then, to indicate the presence of water molecules, you use a prefix that corresponds to the number of water molecules attached to the compound, followed by the word "hydrate." Here are the prefixes for the number of water molecules:

• 1 - Mono-
• 2 - Di-
• 3 - Tri-
• 4 - Tetra-
• 5 - Penta-
• 6 - Hexa-
• 7 - Hepta-
• 8 - Octa-
• 9 - Nona-
• 10 - Deca-

For example, ${\mathrm{CuSO}}_4·{\mathrm{5H}}_{2}O$ is named copper(II) sulfate pentahydrate. The "copper(II)" indicates the oxidation state of copper, "sulfate" is the name of the anion, and "pentahydrate" tells you that there are five water molecules associated with each formula unit of copper(II) sulfate. Remember that the prefix "mono-" is often omitted, so if there's only one water molecule, you would simply write "hydrate" after the compound's name.