Answer:
a) The number density is 3.623 × 10⁻³ 
The mass of the atmosphere is 1.3 × 10²²Kg
b) The pressure is 10⁻²⁰ Millimeter of mercury
c) The mass mixing ratio is 0.0107
The partial pressure of ethane is 0.01114 Pa
Yes it is condensable because it boiling point is -88.5 C which is equivalent to 184.5 K i.e is adding 273 to -88.5C and the temperature of the atmosphere is 37 K.
Explanation:
The explanation is on the first and second uploaded image
Have you ever looked up the density of a substance ? You ought to try it. Go ahead. Pick a substance, then go online or open up an actual book and find its density. You will never see any particular volume mentioned along with the density . . . because it doesn't matter. The whole idea of density is that it describes the substance, no matter how much or how little you have of it. The density of a tiny drop of water under a microscope is the same as the density of a supertanker-ful of water.