I know this is an old post but ima reply anyways.
You know you need gravitational force.
Gravitational force is inversely proportional to the square of the distance so therefore, at a distance of lets say 2x the force will be f/4 (2^2). if it was 3x it would be f/9, at distance r/2 it will be...
I don't know if this is right but this is what i did:
a)
ethylene - C2H4
MM = 28
Amount of C = 24
Amount of H = 4
therefore (780 * 4)/28 = 111.4t
but C2H6 -> C2H4 + H2 (I think)
therefore (1000-780)+111.4 = 331.4t