A student was assigned a colorless unknown. The student added five drops of 1.0 M barium chloride...
Question:
A student was assigned a colorless unknown. The student added five drops of 1.0 M barium chloride solution to 1.0 mL of the unknown solution. A white precipitate formed. In a second experiment, the student added two drops of 0.5 M NH3 solution to 1.0 mL of the unknown solution. A white precipitate formed. The student added five drops of 4.0 M NH3 to the resulting solution in the second experiment. The white precipitate did not dissolve. Identify the colorless unknown solution. Justify your answer with an explanation. Write the net ionic equations for all reactions.
Identifying Solutions Compositions:
Identifying the chemical species in a solution can be simple but also can be very hard. The difficulty of the process depends on how complex the solution is and the nature of the chemical species themselves. For instance, the chemical species could be made of components that precipitate when bonded to a wide range of substances.
Answer and Explanation: 1
The first step the student made was adding barium chloride to the unknown solution. The addition of barium chloride to the unknown resulted in the formation of a white precipitate. This could either mean that the precipitate is the result of the chloride ions bonding with the cations of the unknown or that the precipitate is the result of the barium ions bonding with the anions of the unknown. However, since ammonia was subsequently used twice, barium chloride must have resulted in the precipitation of the anions of the unknown. The reason is that ammonia is used for the detection of certain cations.
Thus, the anion of the unknown is a chemical species that precipitates when bonding with barium ions. According to the solubility rules, the cation could be sulfate ions, carbonate ions, or phosphate ions. As for ammonia, the first time, it caused a precipitate to form, and when excess ammonia was added furthermore, the white precipitate did not dissolve. The first time ammonia was added, the possible cations of the unknown were aluminum ions, lead ions, and zinc ions. But the second addition of ammonia ruled zinc ions out.
Now, the possible cations are aluminum and lead ions, and the possible anions are sulfate ions, carbonate ions, and phosphate ions. The last piece of information that will help us determine the identity of the unknown is the fact that the unknown is soluble, so the unknown compound forms an aqueous solution when added to water. Based on the solubility rules, lead is insoluble when bonded with any of the three anions, so lead ions cannot be the cation of the unknown. Moreover, aluminum is also insoluble when bonded with two of the anions - carbonate and phosphate ions - so aluminum sulfate must be the unknown.
Learn more about this topic:
from
Chapter 3 / Lesson 3Examine five test methods and look at materials used to identify chemicals in a solution. Explore unique methods including precipitation, conduction, and acid and base test, a flame test, and colored solutions used to help identify chemicals.