What is the chemical formula for barium chloride enneahydrate?

Whenever a chemical name is followed by "[prefix]-hydrate", first ignore this last part of the name; hence, write CuCl2. Then, consider that last word - it indicates that there are a number of water molecules corresponding to the prefix - four in this case. The formula is CuCl2 · 4H2O.

FeCr2O7: The two elements chromium and oxygen together form a polyatomic ion, Cr2O7(2-), which is known as the dichromate ion. Since it has a -2 charge, the iron must have a +2 charge. Therefore, the name is iron(II) dichromate.

Barium dihydrogen phosphate: "Dihydrogen phosphate" is a polyatomic ion formed by adding two H+ units to the phosphate ion, PO4(3-). Therefore, the ion has formula H2PO4-. Since barium takes the +2 state, the formula is Ba(H2PO4)2.

More information: http://www.alfa-chemistry.com/cas_13466-20-1.htm

the ennea prefix means '9'.. so I'll take a guess at the formula for barium chloride enneahydrate to be 'BaCl2.9H2O'

 

The Ba ion is 2+ so it needs 2 x Cl- atoms to satisfy it, thus the BaCl2.