A student tests a solution of a sodium salt to see if it contains sulfate ions. They start with 15.0 g of the sodium salt solution in a test tube. First, they add 2.5 g of dilute hydrochloric acid. Next, they add 6 drops of barium chloride solution. Each drop has a mass of 0.15 g. Calculate the total mass of the mixture at the end of this chemical test.