Question

What is the molecular formula of plaster of pairs? (a) \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (b) \(2 \mathrm{CaSO}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) (c) \(2 \mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CaSO}_{4} \cdot 3 \mathrm{H}_{2} \mathrm{O}\)

Short answer

The molecular formula of plaster of Paris is (b) \( 2\text{CaSO}_4 \cdot \text{H}_2\text{O} \).

Step-by-step solution

Understanding Plaster of Paris

Plaster of Paris is a form of calcium sulfate which, when mixed with water, forms a solid hydrate. It is specifically recognized as a hemihydrate form of calcium sulfate, meaning it has half a molecule of water for each molecule of calcium sulfate.

Conceptualizing a Hemihydrate

A hemihydrate can be expressed as \( rac{1}{2} ext{ molecule of water per molecule of Calcium Sulfate} \), which is equivalent to \( ext{CaSO}_4 imes rac{1}{2} ext{H}_2 ext{O} \) when expanded, two units will give us two calcium sulfates combining with one water molecule, \[ 2 ext{CaSO}_4 imes ext{H}_2 ext{O} \].

Key concepts

Plaster of Paris

Plaster of Paris is an interesting and versatile material widely used in different fields. It originates from gypsum, which is a mineral composed mainly of calcium sulfate dihydrate (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). By heating gypsum, most of the water it contains is removed, transforming it into calcium sulfate hemihydrate or Plaster of Paris.When you add water to Plaster of Paris, it reverts to gypsum, hardening into a solid form. This rehydration process makes it incredibly useful for making casts, sculptures, and molds. The result can capture very fine details, which is why it's heavily favored in artistic and medical practices.

Hemihydrate

The term "hemihydrate" refers to a specific condition where a compound contains half a molecule of water for each molecule of another substance. In the context of Plaster of Paris, this translates to its composition having one water molecule bonding with every two calcium sulfate molecules.Hemihydrate notation is represented as:

• The general form: \(\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}\).
• Expanded, it becomes:\[2\text{CaSO}_4 \cdot \text{H}_2\text{O}\].

This means two units of calcium sulfate share a single water molecule. This unique feature helps Plaster of Paris revert to its firm form efficiently when mixed with water. Understanding this concept is key to working with Plaster of Paris effectively, especially for applications requiring precise setting properties.

Calcium Sulfate

Calcium sulfate is a mineral compound with the chemical formula \(\text{CaSO}_4\). It exists naturally as gypsum, the primary source for producing Plaster of Paris. Calcium sulfate is notable for its ability to form various hydrates, with the hemihydrate version being the most relevant in this context.Key points about calcium sulfate include:

• It is stable, non-toxic, and commonly found as gypsum in nature.
• Calcium sulfate, particularly in its hemihydrate form, is useful in a variety of industries such as construction, medicine, and art.
• When in hemihydrate form (Plaster of Paris), it can transition back to dihydrate (gypsum) by adding water, making it versatile for multiple uses.

This compound is crucial not only in forming structures and sculptures but also in creating casts for medical purposes, thanks to its adaptability and strength when dried.