formula and molecular mass

Question

how many grams of MgCl₂ would you need to obtain 5.0 x 10²³ Cl^- ions?

Answer 1

95.2 gmolar mass MgCl2 = 95.2 g/mole

1 mole MgCl2 ==> 2 moles Cl- ion because MgCl2 ==> Mg2+ + 2Cl-

5.0x10²³ Cl^- ions x 1 mole Cl^- ions/6.02x10²³ Cl^- ions = 0.83 moles Cl^- ions

0.83 moles Cl^- ions x 1 mole MgCl₂/2 moles Cl^- ions = 0.42 moles MgCl₂

0.42 moles MgCl₂ x 95.2 g/mole = 40. g MgCl₂ (to 2 significant figures)