The difference of water molecules in Gypsum and Plaster of Paris (POP)? 

We were tasked with determining how the water molecules in plaster of paris and gypsum varied from one another. So let's talk about how plaster of Paris and gypsum are manufactured. The reactions of those two chemicals will then be discussed, and the different water molecule concentrations will be acknowledged.

On the other hand, plaster of Paris is a white powdered slightly hydrated calcium sulphate formed by calcining gypsum and used mostly for castings and moulds in the form of a quick-setting paste with water. Gypsum is a soft sulphate mineral made of calcium sulphate dihydrate. It is produced as a dry powder, which is combined with water just before application to create a hard but workable paste. The crystallisation that results from the interaction with water releases heat, which causes the hydrated plaster to solidify.

Gypsum and plaster of paris are hydrates of calcium sulfate with different number of water of crystallization. 

Gypsum (${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$) is hemihydrate of calcium sulfate and plaster of paris (${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$) is dihydrate of calcium sulfate.

The difference between water molecule becomes;

$2-\frac{1}{2}=\frac{3}{2}=1\frac{1}{2}$

So, option A is correct answer.