A certain reaction requires 0.250 moles of Na2SO4. How much sodium sulfate should the chemist weigh out to perform
the reaction?

The chemist weigh out 35.51 g Na2SO4

Given

0.250 moles of Na2SO4

Required

weight of Na2SO4

Solution

A mole is a number of particles(atoms, molecules, ions)  in a substance

This refers to the atomic total of the 12 gr C-12  which is equal to 6.02.10²³, so 1 mole = 6.02.10²³ particles  

Moles of a substance can also be determined from its molecular weight

For 0.250 moles of Na2SO4, mass =

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.