To calculate the mass of cobalt chloride (CoCl2) obtained when a sample of 20.0 g of cobalt chloride hexahydrate (CoCl2 - 6H2O) is dried in an oven until dry, we need to consider the loss of water molecules from the compound during the drying process.

The formula for cobalt chloride hexahydrate indicates that there are six water molecules associated with each formula unit of CoCl2. When heated, these water molecules are driven off as vapor, leaving behind the anhydrous cobalt chloride.

The molar mass of CoCl2 - 6H2O is 237.93 g/mol, while the molar mass of anhydrous CoCl2 is 129.84 g/mol.

To calculate the mass of CoCl2 in the original sample, we can set up a proportion based on the molar ratio of CoCl2 to CoCl2 - 6H2O:

(Mass of CoCl2) / (Molar mass of CoCl2) = (Mass of CoCl2 - 6H2O) / (Molar mass of CoCl2 - 6H2O)

Substituting the given values:

(Mass of CoCl2) / 129.84 g/mol = 20.0 g / 237.93 g/mol

Solving for the mass of CoCl2:

Mass of CoCl2 = (129.84 g/mol) * (20.0 g / 237.93 g/mol)

= 10.98 g

Therefore, the mass of cobalt chloride obtained after drying the 20.0 g sample of CoCl2 - 6H2O is approximately 10.98 g.