Aluminum has a density of 2.70 g/ml. calculate the mass (in grams) of a piece of aluminum having a volume of 405 ml .

In this question, you are given the density (2.7g/ml) and the volume of the aluminum. You are asked how much the volume is. To answer this question, you need to convert the volume into mass. Since the unit used is already same, you don't need to convert it into another unit. The calculation would be:

mass= volume / density
mass= 405ml / (2.70 g/ml)
mass= 150grams

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.