What is the thermal capacitance of air? Is thermal capacitance and specific heat capacity different?
Thermal capacitance is in Joules/Kelvin while specific heat capacity is in kJoules/kg Kelvin.
If so, how to get the thermal capacitance of air? 
Please give me an example when air is at 250 Kelvin. What is its thermal capacitance?
 A: From Thermal Capacity

Thermal Capacitance
The ability of a material to store heat energy.
Thermal Capacitance is the measure of temperature change in a material based on its volume, as in contrast to Specific heat which is based on the mass of the material.
It is the amount of heat energy in Btu’s absorbed or released by a cubic foot of a material when it temperature changes by 1 °F.

(Obviously this is an out of date quotation, sorry.)

Thermal capacitance depends on the specific heat and the density of a material.

 
Material Thermal Capacitance ( Btu / lb °F )
Air  .018
Water  62.4
Steel  58.7
Copper  50.0
Aluminum 37.5
Silver  36.6
Concrete 21.8
A: If I remember correctly, thermal capacitance is basically a property of a specific system, where specific heat capacity is a property of a material. This is why the units of thermal capacitance is just joules/kelvin instead of kJoules/(kg*Kelvin). To find the thermal capacitance of air, you need to know either the volume of the air and the density or just knowing the mass of system and multiply it by the specific heat(for a single material system).  For a multi material system, the simplest way to figure it out is to just do an experiment.
