(i) The difference of water molecules in gypsum and plaster of Paris is

Gypsum is a naturally occurring mineral composed of calcium sulfate dihydrate (CaSO4·2H2O). This means that there are two water molecules (H2O) attached to each calcium sulfate unit.

On the other hand, Plaster of Paris is a form of gypsum that has been heated to remove the water molecules. Specifically, it is calcium sulfate hemihydrate (CaSO4·0.5H2O). This means that there is only half a water molecule attached to each calcium sulfate unit.

So, the difference in water molecules between gypsum and Plaster of Paris is that gypsum has two water molecules per calcium sulfate unit, while Plaster of Paris has half a water molecule per calcium sulfate unit.