When sodium bicarbonate and acetic acid are combined in a beaker and allowed to
run to completion, a small amount of sodium bicarbonate remains unreacted at the
bottom of the beaker at the end of the experiment. This means that the sodium
bicarbonate is the
in this experiment.
limiting reactant
O excess reactant
O product
catalyst

The mass has escaped into a gas which you can not see so the law stays true.

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.