Question

Supply a systematic name for each of the following hydrate compounds: (a) \(\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Co}(\mathrm{CN})_{3} \cdot 3 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{MnSO}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \cdot 2 \mathrm{H}_{2} \mathrm{O}\)

Short answer

(a) Magnesium sulfate heptahydrate; (b) Cobalt(III) cyanide trihydrate; (c) Manganese(II) sulfate monohydrate; (d) Sodium dichromate dihydrate.

Step-by-step solution

Identify the Anhydrous Compound

For each hydrate, ignore the water molecules temporarily and identify the chemical formula of the anhydrous (water-free) compound. (a) For \( \mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O} \), the anhydrous compound is \( \mathrm{MgSO}_{4} \), known as magnesium sulfate.(b) For \( \mathrm{Co} (\mathrm{CN})_{3} \cdot 3 \mathrm{H}_{2} \mathrm{O} \), the anhydrous compound is \( \mathrm{Co} (\mathrm{CN})_{3} \), known as cobalt(III) cyanide.(c) For \( \mathrm{MnSO}_{4} \cdot \mathrm{H}_{2} \mathrm{O} \), the anhydrous compound is \( \mathrm{MnSO}_{4} \), known as manganese(II) sulfate.(d) For \( \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \cdot 2 \mathrm{H}_{2} \mathrm{O} \), the anhydrous compound is \( \mathrm{Na}_{2} \mathrm{Cr}_2 \mathrm{O}_7 \), known as sodium dichromate.

Specify the Number of Water Molecules

For each hydrate, indicate the number of water molecules attached using the appropriate Greek prefix. (a) The compound contains 7 water molecules, so we use the prefix 'hepta'. (b) The compound contains 3 water molecules, so we use the prefix 'tri'. (c) The compound contains 1 water molecule, so we use the prefix 'mono'. However, the 'mono' is often omitted. (d) The compound contains 2 water molecules, so we use the prefix 'di'.

Combine the Anhydrous Name with the Hydrate Prefix

Combine the name of the anhydrous compound with the hydrate prefix to form the full systematic name of the hydrate.(a) \( \mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O} \) is magnesium sulfate heptahydrate.(b) \( \mathrm{Co} (\mathrm{CN})_{3} \cdot 3 \mathrm{H}_{2} \mathrm{O} \) is cobalt(III) cyanide trihydrate.(c) \( \mathrm{MnSO}_{4} \cdot \mathrm{H}_{2} \mathrm{O} \) is manganese(II) sulfate monohydrate.(d) \( \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \cdot 2 \mathrm{H}_{2} \mathrm{O} \) is sodium dichromate dihydrate.

Key concepts

Understanding Anhydrous Compounds

In chemistry, an anhydrous compound is one where all water molecules have been removed. This is a crucial starting point when dealing with hydrates. Hydrates are substances that include water molecules in their crystalline structure. To systematically name a hydrate, we first identify its anhydrous form. For instance, in magnesium sulfate heptahydrate, the part before the water—magnesium sulfate—is its anhydrous compound, represented by the formula \( \mathrm{MgSO}_{4} \). Removing the water molecules allows chemists to focus on the core substance, which remains constant regardless of the hydrate's state. Recognizing these anhydrous compounds is a foundational step in chemical nomenclature as they determine the root name of any hydrate.

Greek Prefixes in Chemistry Naming

Greek prefixes play a pivotal role in naming chemical compounds, especially hydrates. These prefixes are used to indicate the number of specific atoms or molecules in a compound, such as water molecules in hydrates. Here are some common Greek prefixes used in chemistry:

• Mono- (1): Often omitted when naming, but significant, as in monohydrate for single water molecules.
• Di- (2): Indicates two molecules, such as in dihydrate.
• Tri- (3): Used for three molecules, as in trihydrate.
• Tetra- (4): Signifies four molecules.
• Penta- (5): For five molecules.
• Hexa- (6): Used for six molecules.
• Hepta- (7): Denotes seven, as seen in heptahydrate.
• Octa- (8): For eight molecules.

Using these prefixes in chemical nomenclature ensures clarity and consistency, helping chemists understand the structure and composition of a compound at a glance.

Chemical Nomenclature Rules for Hydrates

Chemical nomenclature rules provide a standardized way to name compounds so that chemists can communicate universally without ambiguity. For hydrates, these rules involve both the identification of the anhydrous compound and the application of Greek prefixes.
To name a hydrate systematically, follow these steps:

• Identify the anhydrous part of the formula. For instance, in \( \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \cdot 2 \mathrm{H}_{2} \mathrm{O} \), the anhydrous compound is \( \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) or sodium dichromate.
• Determine the number of attached water molecules and select the appropriate Greek prefix. In our example, '2' translates to 'di-' for dihydrate.
• Combine the name of the anhydrous compound with the hydrate prefix. Sodium dichromate dihydrate is the full systematic name. This methodical approach follows universal chemistry guidelines and promotes accuracy in scientific communication.

These consistent naming rules facilitate understanding and ensure that each compound is perceived in the same way by chemists worldwide.