Calculate the volume of O2 evolved &mass of KCl formed by decomposition of 30.8 gm KClO3
30.8g = 0.251 moles KClO3
You can see from the reaction
2KClO3 = 2KCl + 3O2
that 1.5 moles of O2 are released for every mole of KClO3. So, you have 0.377 moles of O2.
You know that at STP 22.4 moles of a gas occupy 22.4L of volume.
You can finish it off, right?