Question

Provide the chemical formula for each of the following hydrate compounds: (a) sodium carbonate decahydrate (b) nickel(II) nitrate hexahydrate (c) cobalt(III) iodide octahydrate (d) chromium(III) acetate monohydrate

Short answer

(a) Na₂CO₃·10H₂O; (b) Ni(NO₃)₂·6H₂O; (c) CoI₃·8H₂O; (d) Cr(C₂H₃O₂)₃·H₂O.

Step-by-step solution

Identify the Anhydrous Compound

To find the chemical formula for each hydrate, first identify the anhydrous part of the compound. This is the part of the compound that does not include water molecules. (a) Sodium carbonate is Na₂CO₃. (b) Nickel(II) nitrate is Ni(NO₃)₂. (c) Cobalt(III) iodide is CoI₃. (d) Chromium(III) acetate is Cr(C₂H₃O₂)₃.

Determine the Number of Water Molecules

Next, use the name of the hydrate to determine how many water molecules are associated with each formula unit in the compound. (a) Decahydrate means 10 water molecules. (b) Hexahydrate means 6 water molecules. (c) Octahydrate means 8 water molecules. (d) Monohydrate means 1 water molecule.

Construct the Hydrate Formula

Combine the anhydrous compound and the determined number of water molecules to form the complete hydrate formula. Water molecules are represented by ·nH₂O, where n is the number of water molecules. (a) Sodium carbonate decahydrate: Na₂CO₃·10H₂O. (b) Nickel(II) nitrate hexahydrate: Ni(NO₃)₂·6H₂O. (c) Cobalt(III) iodide octahydrate: CoI₃·8H₂O. (d) Chromium(III) acetate monohydrate: Cr(C₂H₃O₂)₃·H₂O.

Key concepts

Hydrates

Hydrates are fascinating chemical compounds that have a unique feature - they contain water molecules within their structure. This incorporated water is not part of the main chemical frame, but rather, it is bound to the compound in a specific ratio. Hydrates are typically crystalline solids, and the water inside them is known as water of crystallization.

Hydrates are identified by their structural composition. For instance, when looking at a chemical name such as sodium carbonate decahydrate, the suffix '-hydrate' indicates the presence of water molecules. The prefix often gives us valuable information about how many water molecules are present with each formula unit of the compound:

• Decahydrate means 10 water molecules.
• Hexahydrate means 6 water molecules.
• Octahydrate means 8 water molecules.
• Monohydrate means 1 water molecule.

Therefore, by understanding these naming conventions, students can identify how many water molecules are in a hydrate.

Anhydrous Compounds

Anhydrous compounds are the dry counterparts of hydrates. When hydrates lose their water molecules, they become anhydrous. The term 'anhydrous' literally means 'without water.' These compounds do not contain any water of crystallization that hydrates possess.

During the transition from hydrated to anhydrous state, a compound undergoes a process called dehydration, usually through the application of heat. This is often used in industries to remove water from compounds to improve storage and stability.

Knowing the anhydrous form of a compound is crucial for understanding hydrates. For example, in the case of sodium carbonate decahydrate, the anhydrous form is simply sodium carbonate, which has the formula Na₂CO₃. Each of the formulas started with identifying the anhydrous component before figuring out the hydrate through adding the water molecules.

Water Molecules

Water molecules have a special role in chemistry, especially inside hydrates. The water in a hydrate is not just incidental moisture, but is chemically bonded to the compound. Each water molecule adheres to the compound in a fixed proportion, which is usually specified in the compound's name as a numerical prefix.

Understanding the role of water molecules helps students appreciate how hydrates are structured. For each type of compound, the specific arrangement and number of water molecules can significantly alter its physical properties, such as solubility, stability, and even color.

The chemical formula of a hydrate reflects its molecular structure: for instance, in Ni(NO₃)₂·6H₂O, the 6H₂O indicates six water molecules bound to nickel(II) nitrate. This format ("·nH₂O") is crucial for chemists to understand the full composition of hydrates, as it tells how many water molecules exist with each formula unit of the compound.