43.0 g of gypsum, $CaSO X_{4} \cdot 2 H X_{2} O$ , is heated at 393 K to give plaster of Paris quantitatively. The mass of water that must be added to set all the plaster to gypsum again is (Ca 40, S 32, O 16, H 1)

Answer: D

Molar mass of gypsum is

136 + 36 = 172

, so

43.0/172 = 0.250

mol, which gives 0.250 mol of

(CaSO X_{4}) X_{2} \cdot H X_{2} O

-type units, one

CaSO X_{4}

each. Setting takes up 1.5 mol water per mole of

CaSO X_{4}

(from 0.5 to 2). Water

= 1.5 \times 0.250 \times 18 = 6.75

g, which equals the water lost on heating.