SOMEONE PLS HELP ME I DONT UNDERSTAND THIS

Kc = 0.040 for the system below at 450oc. if a reaction is initiated with 0.40 mole of cl2 and 0.40 mole of pcl3 in a 2.0 liter container, what is the equilibrium concentration of cl2 in the same system? pcl5(g) ⇄ pcl3(g) + cl2(g)

What is the molar mass of a gas that has density of 2.054 g/l

What is a possible quantum number set for an electron in the 3s orbital of a magnesium atom