Iupac name for this?

Silica, sio2, is formed on silicon as an electrically insulating layer for microelectronic devices. silica is formed when silicon is exposed to o2 gas at an elevated temperature. at 900˚c, it takes 90 minutes for the oxygen to diffuse from the surface to form a 0.06 micron (0.06 x 10-6 m) thick layer of sio2 on

Iknow the answer to 13 is b and 14 is d. i just need to know why the correct answers are correct

What does x represent in the formula for the compound xcl4?