Question

An 11.75 g sample of a common hydrate of cobalt(ii) chloride is heated. after heating, 9.25 g of anhydrous cobalt chloride remains. what is the formula and name for this hydrate?

Answer 1

Hydrated salts are when salt crystals have water molecules bound. Anhydrous salts are when the water has been removed.
mass of water removed = hydrated salt - anhydrate salt 
                                       = 11.75 g - 9.25 g = 2.50 g
number of water moles  = 2.50 g / 18 g/mol = 0.139 mol 
number of cobalt (II) chloride moles = 9.25 g / 130 g/mol = 0.0712 mol 
ratio of water moles to CoCl₂ moles - 0.139 mol / 0.0712 mol = 1.95 
rounded off 2 moles of water for every 1 mol of CoCl₂
formula - CoCl₂.2H₂O
name - Cobalt(II) chloride dihydrate