Apparently, my answer of .00840 is incorrect, according to my instructor who took a look at my response and told me I had a miscalculation. Not sure where I went wrong, but I was certain it was correct, given that it was listed as an answer choice and other students got the same number.
Calculate the mole fraction of total ions in an aqueous solution prepared by dissolving 0.400 moles of MgCl2 in 850.0 g of water.