What volume of oxygen is needed to react with a solid sulfur to form 3.5L of SO2?

1 mole of gas occupies 22.4 liters at STP. Therefore 3.5/22.4 = 0.15625 moles of SO2.

There are thus 0.15625 moles of O2 needed to react with solid sulfur

because S + O2 ---->SO2.

0.15625 moles of oxygen occupies 0.15625 x 22.4 liters = 3.5 liters O2 required.