Q7 / 160 · TS EAMCET 2004 · Chemistry

In the hardening stage of plaster of paris, the compound formed is

  • A
    $\mathrm{CaSO}_4$
  • B
    $Orthorhombic$ $\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$
  • C
    $\mathrm{CaSO}_4 \cdot \mathrm{H}_2 \mathrm{O}$
  • Correct answer
    D
    $Monoclinic$ $\mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O}$

Solution

Plaster of paris absorb water to form monoclinic gypsum which is a hard substance. $$ \mathrm{CaSO}_4 \cdot \frac{1}{2} \mathrm{H}_2 \mathrm{O} \stackrel{3 / 2 \mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} $$ (monoclinic gypsum)

Hence, the correct answer is option: 'D'.

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