How do you balance #O_2 + H_2S -> H_2O + S#?
This is a double displacement reaction. It can be balanced by ensuring that there are the same number of molecules on both sides.
First of all, we are going to need at least two molecules of oxygen on the right side. This means we will end up with 4 molecules of hydrogen, which gives us the following:
Since we have four molecules of hydrogen on the right side, we need four molecules of hydrogen on the left as well:
Seeing that we have four molecules of hydrogen on the left, we have two molecules of sulphur on the left as well.
This is totally balanced, since there is the same amount of everything on the left and right sides.
Hopefully this helps!