Suppose you had one mole of oxygen (o2). how many moles of hydrogen (h2) would react completely with the oxygen, and how many moles of h2o would be produced?

2H₂  +  O₂  >  2H₂O

mole ratio of  O₂  :  H₂  =  1 : 2

since mole of O₂ = 1
   then mole of H₂  =  1`* 2
                              =  2

since mole ratio of  H₂  :  H₂O  =  2 : 2
                    then mole of H₂O  =  2

The answer is c. 320 g

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.