Calculate the molarity of the solution. use the periodic table if necessary. a salt solution with a volume of 250 ml that contains 0.70 mol nacl.

Molarity is a way to express the concentration of the solution. It is defined as the number of moles of solvent per one liter of solution. 
Molarity = number of moles / volume in liter.
The number of moles = 0.70 mol as given. The volume = 250 ml = 250/1000 liter = 0.25L.
Molarity = 0.70/0.25 = 2.8M

The correct answer is "atomic weight".