Question

1. For each of the descriptions of different solutions, what inferences can be made? (Which ion(s) are identified as possibly present, confirmed present, absent, or no conclusion?)

a. Hot water was added to a white precipitate (AgCl and/or PbCl2). After stirring, the precipitate completely dissolved forming a colorless solution.

b. Aqueous ammonia was added to a solution (Cu2+ and/or Bi3+). After stirring, a white precipitate formed, with a colorless solution above it.b.

c. HCl was added to Group 1 and 2 unknown (Ag+, Pb2+, Cu2+, and/or Bi3+). After stirring, some white precipitate formed and the solution above the precipitate was colorless.

d. Aqueous ammonia was added to a solution (Cu2+ and/or Bi3+). After stirring, a dark blue solution formed with no precipitate.

2. A student performing the Qual Group 1 and 2 unknown analysis did not have any precipitate remaining after step 1-A. What steps should the student omit from the procedure? Explain your answer.

Answer 1

1. Conclusions

a. PbCl2 is soluble in hot water, AgCl is insoluble in hot water : Thus PbCl2 is present

b. Bi3+ forms the insoluble hydroxide with aq. NH3 : Thus Bi3+ is present

c. Group I cations form insoluble chlorides upon addition of dil HCl : thus Ag+ and Pb2+ are present

d. Cu2+ forms ammonia complex with excess NH3 added which is blue in color : Thus Cu2+ is present

2. The student should avoid adding hot water to the precipitate mixture so that the formed precipitate does not dissolve and we could isolate it for further characterisation.