Which equation shows the proper setup to find the volume of a 0.250 m cacl2 solution that contains 39.5 g of cacl2?

Molarity = number of mole of substance(n) / volume of solution (V).

n(CaCl2) = mass (CaCl2)/M(CaCl2)

M(CaCl2) = 40+2*35.5 = 111 g/mol 

n(CaCl2) =39.5 g CaCl2*1 mol/111g

0.250 M = 39.5 g CaCl2*1 mol/111g*volume of solution (V).

volume of solution (V) = 39.5 g CaCl2*1 mol/(0.250 M*111g) = 1.42 L

The density would be 1.622

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.