There are 6.02E23 molecules in 1 mol CaCl2. So there must be 6.02E23 x 2.35 = ? molecules in 2.35 instead of 1 mol of CaCl2. Then the formula tells you that there are 2 Cl ions in 1 molecule so the number of Cl^- must be twice the number of molecules of CaCl2.
