Na2so4 is dissolved in water to make 450 ml of a 0.250 m solution. what is the molar mass of na2so4, and how many moles are needed?

Molarity is Given as,

                       Molarity  =  Moles / Volume of Solution  --- (1)

Data Given:
                   Molarity  =  0.250 mol.L⁻¹

                   Volume  =  450 mL  =  0.45 L

Solving equation 1 for Moles,

                    Moles  =  Molarity × Volume

Putting values,

                    Moles  =  0.250 mol.L⁻¹ × 0.45 L

                    Moles  =  0.1125 mol

Now, converting moles into grams,
As,
                    Moles  =  Mass / M.mass
Or,
                    Mass  =  Moles × M.mass

                    Mass  =  0.1125 mol × 142 g.mol⁻¹

                    Mass  =  15.97 g

Result:
            Molar Mass of Na₂SO₄  =  142 g.mol⁻¹

            Moles needed  =  0.1125 mol

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.