Consider N free particles wi which the energy of each particle can assume two and only two distinct values , O and E (E> o ) . Denote by no and me the number of particles with energy 0 and energy E (a) find the entropy of the system (b) find the most probable values of Mo and n, and their mean square fluctuation (c) find the temperature as a function of the total energy U and show that - it can be negative

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant. 

And a) oxygen i believe

I'm pretty sure the answer is alcohol