$$ extbf{Part (1): Name of the water molecules present in the salt}$$ The chemical name of hydrated calcium sulphate $$(CaSO_{4}.2H_{2}O)$$ is calcium sulphate dihydrate. It is commonly known as Gypsum. The name given to the water molecules present in calcium sulphate salt is the "water of crystallization". $$ extbf{Part (2): Percentage of water molecules in hydrated calcium sulphate}$$ The molecular mass of $$CaSO_{4}.2H_{2}O$$ can be calculated as, $$egin{align*}CaS{O_4}.2{H_2}O &= (1 imes Ca) + (1 imes S) + (4 imes O) + (4 imes H) + (2 imes O)\\ &= (1 imes 40) + (1 imes 32) + (4 imes 16) + (4 imes 1) + (2 imes 16)\\ &= 40 + 32 + 64 + 4 + 32\\ &= 172end{align*}$$ The molecular mass of water of crystallization present in salt can be calculated as, $$egin{align*}2{H_2}O &= 2left( {2 imes H + 1 imes O} \right)\\ &= 2left( {2 + 16} \right)\\ &= 36end{align*}$$ The percentage of water molecules in hydrated calcium sulphate can be calculated as shown below. $$egin{align*}% ;of;water;of;crystallization &= dfrac{{Mass,of;water}}{{Total;mass;of;hydrated;salt}} imes 100% \\ &= dfrac{{36}}{{172}} imes 100% \\ &= 20.93% end{align*}$$ Therefore, the percentage of water molecules in hydrated calcium sulphate is $$20.93 %.$$