A6.53 g sample of a mixture of magnesium carbonate and calcium carbonate is treated with excess hydrochloric acid. the resulting reaction produces 1.73 l of carbon dioxide gas at 26 ∘c and 745 torr pressure.
a)calculate the total number of moles of carbon dioxide that forms from these reactions.
express your answer using three significant figures.
b) assuming that the reactions are complete, calculate the percentage by mass of magnesium carbonate in the mixture.

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. 

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.