Question

Write the name from the formula or the formula from the name for each hydrated ionic compound. \begin{equation} \begin{array}{l}{\text { a. } \operatorname{cobalt}(\mathrm{II}) \text { phosphate octahydrate }} \\ {\text { b. } \mathrm{BeCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}} \\ {\text { c. chromium(III) phosphate trihydrate }} \\ {\text { d. LiNO }_{2} \cdot \mathrm{H}_{2} \mathrm{O}}\end{array} \end{equation}

Short answer

a. Co3(PO4)2 · 8H2O, b. Beryllium chloride dihydrate, c. CrPO4 · 3H2O, d. Lithium nitrite monohydrate.

Step-by-step solution

Determine the formula for cobalt(II) phosphate octahydrate

Identify the ions involved: cobalt(II) means Co with a 2+ charge and phosphate is PO4 with a 3- charge. Balance the charges to get a neutral compound. The formula Co3(PO4)2 indicates one ion of cobalt for every two ions of phosphate, meaning three cobalt ions are needed to balance two phosphate ions. To indicate octahydrate, 8 water molecules are attached: Co3(PO4)2 · 8H2O.

Name the compound BeCl2 · 2H2O

BeCl2 is beryllium chloride, and the 2H2O indicates it is a dihydrate. Thus, the name of the compound is beryllium chloride dihydrate.

Determine the formula for chromium(III) phosphate trihydrate

Chromium(III) indicates a Cr ion with a 3+ charge, and phosphate is PO4 with a 3- charge. The charges are already balanced, so one ion of each gives: CrPO4. For the trihydrate portion, attach 3 water molecules: CrPO4 · 3H2O.

Name the compound LiNO2 · H2O

LiNO2 is lithium nitrite. Given that it has one water molecule attached, this makes it a monohydrate. Therefore, the name is lithium nitrite monohydrate.

Key concepts

Naming Ionic Compounds

Understanding how to name ionic compounds is essential for chemistry students. The process involves identifying the cation (positively charged ion) and the anion (negatively charged ion). The cation is always named first, followed by the anion. If the cation is a metal that can form ions with different charges, such as cobalt in the example, the charge is indicated in Roman numerals right after the metal's name, like cobalt(II).

For anions that are single elements, the end of the element’s name is typically replaced with the suffix '-ide.' However, more complex anions are called polyatomic ions and have unique names, such as 'phosphate' for PO4^3-. By applying these rules, 'cobalt(II) phosphate' refers to a compound made of Co²⁺ and PO4^3- ions. When hydrates are involved, the number of water molecules is denoted by a prefix (mono-, di-, tri-, etc.) followed by the term 'hydrate.' For example, 'cobalt(II) phosphate octahydrate' includes eight water molecules.

Chemical Formulas

Chemical formulas express the types and numbers of atoms in a compound, allowing chemists to understand a substance’s composition at a glance. The formula for an ionic compound includes symbols for the cations and anions, followed by subscripts that reflect the ratio needed to neutralize the compound’s charge. For instance, in the example of beryllium chloride, BeCl2, the subscript '2' indicates two chloride ions for each beryllium ion.

When denoting the hydration of a compound, the formula is followed by a dot and the corresponding number of water molecules. This is symbolized as '· nH2O,' where 'n' is the number of water molecules. The formula BeCl2 · 2H2O means two water molecules are associated with each beryllium chloride unit, classifying it as a dihydrate.

Compound Hydration

Compound hydration indicates the presence of water molecules bound within a crystal structure of an ionic compound. These water molecules are an integral part of the compound's solid state and can affect its properties. The notation of hydration follows the stoichiometric formula with a dot, representing how these water molecules systematically associate with the compound.

The prefixes ‘mono-’, ‘di-’, ‘tri-’, etc., correspond to the number of water molecules. For example, chromium(III) phosphate trihydrate is written as CrPO4 · 3H2O, signaling three molecules of water per formula unit. Accurate identification and communication of hydration levels are important, especially since different levels of hydration can result in substances with distinct physical characteristics.

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, that behave as a single unit with a net charge. They are common in various types of chemical compounds, including hydrated ionic compounds. Examples are the phosphate ion (PO4^3-) and the nitrite ion (NO2^-). Understanding their structures and names is crucial in naming compounds correctly.

These polyatomic ions are recognized by specific names and formulas that must be memorized, as they do not conform to standard naming rules based on their component elements. For instance, in the examples provided, the compounds contain 'phosphate' and 'nitrite,' which are indicative of the PO4^3- and NO2^- ions, respectively. Recognizing these ions allows for proper naming of the compounds, like lithium nitrite monohydrate (LiNO2 · H2O).