An archer pulls back the string of the bow (spring constant of 250 N/m) a distance of 1 meter to shoot a 0.05 kg arrow. What is the maximum speed of the arrow as it leaves the bowstring?

The formula is CaCl2 as you need two Cl ions in order to combine with one Ca ion in order to form a neutral compound i.e. You need to have the same amount of charges therefore  you have two Cl ions which have  -1 charges for one +2 Ca ion.
Hope this helps :).