How many grams of potassium chloride is produced if 6.78 mols of potassium reacts?

Potassium   react  with  chlorine  to  form  potassium  chloride
2K +  Cl2  ---> 2 KCl
The  reacting  ratio  of   potassium  reacted  and  KCl  produced  is  1:1
by  use  of  this  ratio  the number  of  moles  of  KCl  produced  is  therefore =6.78moles
mass =molar  mass  x  number of  moles
molar  mass  of  KCl=74.5  g/moles
mass  is  therefore=  74.5 x  6.78moles=505.11g

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.