Plaster of Paris (${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$) sets into a hard solid mass on mixing with water. Write the balanced chemical equation for this setting reaction. Also, calculate the percentage of water of crystallisation by mass present in hydrated copper sulphate (${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$). (Given atomic masses: $\text{Cu}=63.5\text{u}$, $\text{S}=32\text{u}$, $\text{O}=16\text{u}$, $\text{H}=1\text{u}$)

Step-by-step solution

1. Step 1: Write the equation for the setting of Plaster of Paris into gypsum. ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$.
2. Step 2: Calculate the molar mass of hydrated copper sulphate, ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. Molar mass $=63.5+32+(4\times 16)+5\times [(2\times 1)+16]=63.5+32+64+5\times 18=159.5+90=249.5\text{g/mol}$.
3. Step 3: Calculate the mass of water of crystallisation in one mole of ${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$. Mass of water $=5\times 18=90\text{g/mol}$.
4. Step 4: Calculate the percentage of water by mass using the formula: $\text{Percentage}=\left(\frac{\text{Mass of }5{\text{H}}_2\text{O}}{\text{Molar mass of }{\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}}\right)\times 100=\left(\frac{90}{249.5}\right)\times 100\approx 36.07\%$.

Concept tested: Plaster of Paris and water of crystallisation calculation

Published 24 August 2026 · Last updated 16 September 2026 · How this solution was prepared and checked

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