At a certain temperature, the equilibrium constant, K c , Kc, for this reaction is 53.3. H 2 ( g ) + I 2 ( g ) − ⇀ ↽ − 2 HI ( g ) K c = 53.3 H2(g)+I2(g)↽−−⇀2HI(g)Kc=53.3 At this temperature, 0.400 mol H 2 0.400 mol H2 and 0.400 mol I 2 0.400 mol I2 were placed in a 1.00 L container to react. What concentration of HI HI is present at equilibrium?

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. 

Density and pressure

Ido not understand.