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Gypsum, known chemically as calcium sulfate dihydrate (\(\text{CaSO}_4 \, \cdot \, 2\text{H}_2\text{O}\)), undergoes dehydration when subjected to heat. When heated, gypsum loses water and transforms into different states depending on the temperature. This process is important in various industrial applications, especially in construction materials like plaster.At around 433 K, gypsum loses one and a half molecules of water per formula unit, converting into the hemihydrate form (\(\text{CaSO}_4 \, \cdot \, \frac{1}{2}\text{H}_2\text{O}\)).This substance is also known as plaster of Paris. The formation of plaster of Paris is a partially dehydrated form which rehydrates when water is added, making it a popular choice for casting molds among other uses.
At a higher temperature of 463 K, the hemihydrate further loses water, resulting in the creation of \(\gamma\)-\text{CaSO}_4, an anhydrous form of gypsum. This further dehydration represents a continuation of the removal of bound water, shifting the material to a more dry and less reactive state.

Calculating the molar mass is a vital step in determining the changes in a compound's weight during chemical reactions like dehydration.Molar mass refers to the mass of one mole of a given substance, expressed in grams per mole (g/mol).To calculate the molar mass of gypsum, we sum the atomic masses of each element present:

• Calcium (Ca): 40.08 g/mol
• Sulfur (S): 32.07 g/mol
• Oxygen (O): 16.00 g/mol – with four atoms per molecule of \(\text{SO}_4\)
• Water (\(\text{H}_2\text{O}\)): 18.016 g/mol per molecule – doubled for the gypsum formula

By combining these values, the molar mass of gypsum is given by:\[40.08 + 32.07 + 4(16.00) + 2(18.016) = 172.17 \text{ g/mol}.\]This calculated figure allows us to further understand and compute weight changes during the dehydration process. Similarly, calculating the molar masses of the subsequent dehydrated forms (hemihydrate and anhydrite) enables comparison and understanding of the material conversions.

Weight percentage change is an essential calculation in thermogravimetric analysis (TGA), reflecting how much mass a compound loses due to a change, such as temperature-induced dehydration.This is calculated by examining the difference in molar masses before and after the intervention.Let us consider the example of gypsum dehydrating to form the hemihydrate:1. **Determine Weight Loss:** - Initial molar mass of gypsum: 172.17 g/mol. - Molar mass of hemihydrate: 145.15 g/mol.
The difference is \[ 172.17 - 145.15 = 27.02 \text{ g/mol}. \] This indicates a weight loss of 27.02 g/mol in transforming to hemihydrate.2. **Calculate Percentage Weight Change:** Using the formula: \[ \text{Percentage weight change} = \left( \frac{27.02}{172.17} \right) \times 100 \approx 15.7\%. \] This means a 15.7% reduction in mass as gypsum transitions to the hemihydrate form.Finally, when the hemihydrate reaches higher temperatures to become \(\gamma\)-\text{CaSO}_4, another calculation is done to find the total weight change from the original gypsum, showcasing a greater overall percentage weight change. Understanding these calculations is crucial in interpreting TGA curves and predicting material behavior during thermal treatments.