_Cl2O5 + _H2O → _HClO3
From the H2O, we see that HCLO3 will have to have a multiple of 2 in front, since we only get delivered an even number of oxygen atoms:
_Cl2O5 + _H2O → 2HClO3
Now, on the right side side we have 2 H atoms (same as on the left side), 2 Cl atoms (again, the same) and 6(2*3) oxygen atoms (again the same: 5 from the first part and one from the second.
So this is already our solution:
Cl2O5 + H2O → 2HClO3
