Question

how many liters of oxygen are required to react completely with 12.5 liters of hydrogen sulfide at stp?

Answer 1

You need an equation and it must be balanced.
Convert 12.5 L H2S to mols. That is 12.5L/22.4 = ? mols H2S.

Use the coefficients in the balanced equation to convert mols H2S to mols Oxygen.

Now convert mols oxygen to L.
mols O2 x 22.4L= ?

There is a shortcut you can use if you wish. Since all are gases, you may omit the mols step at the front and back and just convert L H2S to L O2 directely using the coefficients as above