What is the concentration of the iodide atom...?

Okay, after you have figured out your chemical equation, you then find how many grams are in 1 mol of CaI2, which is 293.88g. Covert your given m of 12.2 g into moles by dividing it by the molar m: 12.2 g CaI2*(1mol CaI2/ 293.88g) the grams cancel and you are left with 0.0415 mol CaI2. Now, based on the balanced equation, you find the molar ratio of CaI2 to I- is 1:2. --> 0.0415 mol CaI2*(2 mol I-/1 mol CaI2) = 0.083 mol I-. Now convert to M by dividing the moles of I- by the total volume in L. 500 mL=0.5L --> (0.083 mol I-/0.5 L)= 0.166 M I-.