To test the purity of calcium chloride you dissolve a 3.77g sample in water and add silver perchlorate to precipitate out silver chloride.?

By adding silver perchlorate, any chloride ions present in the solution will react with the silver ions to form silver chloride precipitate. The mass of silver chloride can be measured to determine the amount of chloride present, which can then be used to calculate the purity of the calcium chloride sample.