Calculate the molar mass of each hydrated compound. Note that the water of hydration is included in the molar mass. 

Given Data 

(a). H₂C₂O₄ ∙ 2 H₂O 

(b). MgSO₄ ∙ 7 H₂O, Epsom salt

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A CLOSER LOOK Hydrated lonic Compounds If ionic compounds are prepared in water solution and then isolated as solids, the crystals often retain mole- cules of water. These compounds are called hydrated compounds. For ex- ample, crystals of the red cobalt(II) compound in the Figure have six wa- ter molecules per CoCl₂. By conven- tion, the formula for this compound is written as CoCl₂-6 H₂0. The dot be- tween CoCl₂ and 6 H₂O indicates that 6 molecules of water are associated with every CoCl₂. The name of the compound is cobalt(II) chloride hexahydrate. Hydrated cobalt(II) chloride, the red solid in the Figure, turns purple and then deep blue as it is heated and loses water to form anhydrous CoCl₂; "anhydrous" means without water. On exposure to moist air, anhydrous CoCl₂ takes up water and is converted back into the red hydrated com- pound. It is this property that allows crys- tals of the blue compound to be used as a humidity indicator. You may have seen them in small bags packed with a piece of electronic equipment. Hydrated compounds are common. The walls of your home are probably covered with wallboard, or "plaster board," which contains hydrated calcium sulfate (gypsum, Cobalt(II) chloride When it is heated, hexahydrate (CoCl₂ + 6H₂O] the compound loses is deep red. some of the water of hydration. CaSO4 2 H₂O), as well as anhydrous CaSO4, sandwiched between paper. If gypsum is heated between 120 and 180 °C, the water is partly driven off to give CaSO4 1/2 H₂O, a compound commonly called "plaster of Paris." If you have ever broken an arm or leg and had to have a cast, the cast may have been made of this compound. It is an effec- tive casting material because, when added Photos Cengage Learning/Charles D. Winters Heating ultimately leads to the deep blue compounds CoCl₂.2 H₂O and CoCl₂. Experiments show that some also decomposes to black CoO and HCI. to water, it forms a thick slurry that can be poured into a mold or spread out over a part of the body. As it takes on more water, the material increases in volume and forms a hard, inflexible solid. Plaster of Paris is also a useful material for artists, because the expanding compound fills a mold completely and makes a high-quality reproduction.