Question

Provide the chemical formula for each of the following hydrate compounds: (a) sodium acetate trihydrate (b) calcium sulfate dihydrate (c) potassium chromate tetrahydrate (d) zinc sulfate heptahydrate

Short answer

(a) \( \text{NaC}_2\text{H}_3\text{O}_2 \, \cdot \, 3\text{H}_2\text{O} \); (b) \( \text{CaSO}_4 \, \cdot \, 2\text{H}_2\text{O} \); (c) \( \text{K}_2\text{CrO}_4 \, \cdot \, 4\text{H}_2\text{O} \); (d) \( \text{ZnSO}_4 \, \cdot \, 7\text{H}_2\text{O} \).

Step-by-step solution

Understand the Basics of Hydrates

Hydrates are compounds that have a specific number of water molecules attached to them. The general formula for a hydrate is given as \( \text{Compound} \, \cdot \, n\text{H}_2\text{O} \), where \( n \) represents the number of water molecules.

Sodium Acetate Trihydrate

Sodium acetate is represented as \( \text{NaC}_2\text{H}_3\text{O}_2 \). In sodium acetate trihydrate, three water molecules are attached, so the formula is \( \text{NaC}_2\text{H}_3\text{O}_2 \, \cdot \, 3\text{H}_2\text{O} \).

Calcium Sulfate Dihydrate

Calcium sulfate is \( \text{CaSO}_4 \). With dihydrate, it means two water molecules are attached. Thus, the chemical formula is \( \text{CaSO}_4 \, \cdot \, 2\text{H}_2\text{O} \).

Potassium Chromate Tetrahydrate

Potassium chromate is \( \text{K}_2\text{CrO}_4 \). Tetrahydrate indicates four water molecules are attached, so the formula is \( \text{K}_2\text{CrO}_4 \, \cdot \, 4\text{H}_2\text{O} \).

Zinc Sulfate Heptahydrate

Zinc sulfate is \( \text{ZnSO}_4 \). Heptahydrate denotes seven water molecules, so the complete formula is \( \text{ZnSO}_4 \, \cdot \, 7\text{H}_2\text{O} \).

Key concepts

Hydrates

In chemistry, hydrates play an essential role in the structure of compounds. A hydrate is a compound that includes water molecules incorporated into its crystal structure. These water molecules are not simply "wet" or loosely attached but are integrated into the compound.
The general formula for hydrates is written as \( \text{Compound} \, \cdot \, n\text{H}_2\text{O} \). Here, "Compound" is the anhydrous or non-water part of the molecule, while \( n \) is the number of water molecules bonded.
Understanding the nature of hydrates is key for various applications, including predicting how substances will behave under different environmental conditions and their utility in industrial processes. Hydrates can alter the physical properties of compounds, like melting points and solubilities.

Sodium Acetate Trihydrate

Sodium acetate trihydrate is a common hydrate form of sodium acetate, which is expressed in its chemical formula as \( \text{NaC}_2\text{H}_3\text{O}_2 \, \cdot \, 3\text{H}_2\text{O} \). This formula indicates that for every unit of sodium acetate, three water molecules are part of the crystal structure.
Sodium acetate is frequently used in heating pads, and the trihydrate form can play a significant role in the release of heat. When these water molecules are removed through heating, sodium acetate goes into a supercooled liquid state, and upon recrystallization, it releases heat, making it useful for heat packs.
The presence of water molecules in crystalline sodium acetate influences its storage and usage, as the compound must be stored in conditions that prevent dehydration.

Calcium Sulfate Dihydrate

Known commonly as gypsum, calcium sulfate dihydrate has a significant place in industries such as construction and art. The chemical formula \( \text{CaSO}_4 \, \cdot \, 2\text{H}_2\text{O} \) shows that each unit of calcium sulfate has two water molecules attached.
In the construction industry, gypsum is used as a plaster material. Upon heating, the dihydrate form converts to hemihydrate, known as plaster of Paris, which is widely used for making molds and sculptures. This transformation illustrates the importance of the dihydrate state, which makes it versatile for different applications depending on the level of hydration.
Understanding the role of the two water molecules is crucial, as their presence affects the durability and strength of the plaster or board made from gypsum.

Zinc Sulfate Heptahydrate

Zinc sulfate heptahydrate is a well-known hydrate, primarily used in supplements and agricultural applications. Its formula, \( \text{ZnSO}_4 \, \cdot \, 7\text{H}_2\text{O} \), indicates that seven water molecules are a part of its structure, which significantly affects its physical properties and solubility in water.
In agriculture, zinc sulfate heptahydrate is used as a micronutrient supplement to enrich soil, assisting in proper plant growth. This form of zinc sulfate provides an effective means of delivering the zinc nutrient due to its water solubility, facilitated by the presence of seven water molecules.
Additionally, this hydrate form can be used in medicine to address zinc deficiencies, demonstrating its importance in health as well as in industrial applications.