What is the mathematical relationship between the variation of thermal conductivity of air & its density ?

Update: An enclosed rectangular chamber with only two of its opposite faces exposed to differential temperatures & other faces heat insulated , contains air at a particular absolute pressure. As the pressure is varied inside the chamber the corresponding variation in its thermal conductivity is to be ascertained. A... show more An enclosed rectangular chamber with only two of its opposite faces exposed to differential temperatures & other faces heat insulated , contains air at a particular absolute pressure. As the pressure is varied inside the chamber the corresponding variation in its thermal conductivity is to be ascertained. A mathematical expression relating to the above is required.
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