Bromime ions (bromide) have oxidation state 1- => Br (-).
So, to be neutral the compound has to have two Br (-) ions per each Ca(2+) ion.
That is represented in the chemical formula as Ca Br2, where the number 2 to the right of Br is a subscript meaning that there are two atoms of Br per each atom of Ca (the lack of subscript means 1 atom).
We calculate for the number of moles of water given its mass by dividing the given mass by the molar mass. n water = (36.04 g) / (18 g/mol) n water = 2 mols From the given balanced equation, every 6 moles of water produced will require 7 moles of oxygen. n oxygen = (2 mols H2O) x (7 moles O2 / 6 moles H2O) n oxygen = 2.33 mols O2
= 71.2 g