Question

why plaster of paris is stored in air tight packs or dry place

Answer 1

because air contains moisture this moisture can cause the setting of plaster of Paris into hard mass gypsum, Which makes the Plaster of Paris useless.
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Answer 2

Plaster of paris is $CaSO_4. \frac{1}{2}H_2O$. It has a tendency to absorb moisture and become gypsum$(CaSO_4.2H_2O)$ which is undesirable. If it absorbs moisture, it becomes hard and it can't be used any more. So to prevent absorption of moisture from air, it is stored in tight packs or dry place.