0.200mol of H2, 0.200mol of I2, and 0.200mol of HI were placed in a 1.00 L flask and allowed to come to equilibrium. The Kc value of the reaction at this temperature is 49.5. Determine the equilibrium concentrations of all species.

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. 

As cooking food that has to go in the oven

That is a lot of moles that you have