A commercial sample (2.0189 gram) of NaOH containing Na2CO3 as impurity was dissolved in water to make 250ml solution . A 10 ml portion of this solution required 20 ml of 0.1 N H2SO4 for complete neutralization. Calculate the percentage by weight of Na2CO3 in the sample.

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. 

No. an alpha particle is a pair of protons and a pair of neutrons, i.e. a helium