What does the term density mean?

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

The solid xy decomposes into gaseous x and y: xy(s) m x(g) + y(g) kp = 4.1 (at 0 °c) if the reaction is carried out in a 22.4 l container, which initial amounts of x and y will result in the formation of solid xy?

Which item is most likely part of the safety contract