Question

Give the chemical formula of Calcium sulphate hemihydrate

Answer 1

Calcium sulphate hemihydrate:

• Calcium sulphate hemihydrate is one molecule of ${\mathrm{CaSO}}_4$containing $\frac{1}{2}$molecule of water and is also known as Plaster of Paris
• Its chemical formula can be written as ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.
• Plaster of Paris is a white powder, which when mixed with water sets into a hard mass and is used for fixing broken bones.
• It is formed by heating gypsum (${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$) at $373–393\mathrm{K}$ or $120°C$.

$\underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{120°\mathrm{C}}{\to }\underset{\mathrm{Plaster}\mathrm{of}\mathrm{Paris}}{\mathrm{C}{\mathrm{aSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}$

Therefore, the chemical formula of Calcium sulphate hemihydrate is ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.