Phosphorus pentachloride, PCl5, is a white solid that sublimes (vaporizes without melting) at about 100oC. At higher

Question:

Phosphorus pentachloride, PCl5, is a white solid that sublimes (vaporizes without melting) at about 100oC. At higher temperatures, the PCl5 vapor decomposes to give phosphorus trichloride, PCl3, and chlorine, Cl2.
PCl5(g) → PCl3(g) + Cl2(g)
How could gas-density measurements help to establish that PCl5 vapor is decomposing? Stoichiometry with Gas Volumes