Chemistry 5.5 Names and Formulas: Hydrates

Summary

This lesson introduces hydrates, a type of ionic compound that traps water molecules within its crystalline structure. It covers how to name hydrates by first naming the ionic compound and then adding a prefix to indicate the number of water molecules, followed by 'hydrate'. The lesson also explains how to write the chemical formula for hydrates given their name, emphasizing the use of a dot to separate the ionic compound from the water molecules.

Highlights

What are Hydrates?00:00:00

Hydrates are ionic compounds or ionic salts that can trap water molecules within their crystalline structure. These trapped water molecules are caged by the positive and negative ions of the salt, leading to the term 'hydrated salts'.

Naming Hydrated Salts00:00:56

To name a hydrate, first name the ionic compound as usual. Then, determine the number of water molecules (H₂O) present and use a numerical prefix (e.g., 'di' for two, 'hepta' for seven) followed by 'hydrate'. The dot between the ionic compound and H₂O in the formula indicates 'contains' and does not mean multiplication.

Example 1: Copper(II) Chloride Dihydrate00:01:15

For CuCl₂·2H₂O, the ionic compound is copper(II) chloride. Since there are two water molecules (2H₂O), the prefix 'di' is used, making the full name copper(II) chloride dihydrate.

Example 2: Zinc Sulfate Heptahydrate00:02:31

For ZnSO₄·7H₂O, the ionic compound is zinc sulfate. With seven water molecules (7H₂O), the prefix 'hepta' is used, resulting in the name zinc sulfate heptahydrate.

Writing Formulas for Hydrates00:03:08

To write the formula for a hydrate, first write the formula for the ionic compound. Then, identify the prefix for 'hydrate' to determine the number of water molecules. Connect the ionic compound formula and the water molecules with a dot (·) to indicate it's a hydrated salt.

Example: Copper(II) Sulfate Pentahydrate00:03:15

For copper(II) sulfate pentahydrate, the ionic compound is copper(II) sulfate, so the formula is CuSO₄. 'Penta' indicates five water molecules (5H₂O). The complete formula is CuSO₄·5H₂O.