Purpose of the Experiment
Determine the presence or absence of each cation in Qualitative Analysis Groups 1 and 2.
Background Required
You should be familiar with techniques found in the Qualitative Inorganic Analysis Preview.
You should have the observations from the Groups 1 and 2 Known for comparisons.
Always Wear Safety Goggles and Use Good Lab Practices
Groups 1 and 2 Unknown Solution
Your lab instructor will issue you an Unknown. Immediately record the number of your Unknown. Then repeat the Groups 1 and 2 procedure, substituting your unknown solution for the known cation mixture in step 1–A1. Carefully record all observations in your lab notebook pages.
Some unknown observations will not exactly match the observations of the Known solution, depending on which cation(s) is/are absent. For example, if you do not have Pb2+ and Ag+ present, you will not observe a white precipitate in step 1–A. And since you do not have a precipitate to test, you will have to omit steps 1–B, 1–C, 1–D, and 1–E. Or if Cu2+ is absent, then the solution in step 1–A will not be blue, but you still have a solution to test and will still do the next step. When in doubt if you should omit a procedural step, check with your instructor.
Determinations
Construct a flowchart for your unknown, writing in your observations under the formulas.
Remember, some parts of the flowchart will be omitted if you had to omit some procedural steps.
Write a rationale explaining how you determined which cations are absent and which are present. Refer to the Qualitative Inorganic Analysis Preview section, to see an example of a Rationale.
1. For each of the descriptions of different solutions, what inferences can be made? (Which ion(s) are indicated 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.
c. HCl was added to a 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.
3. A student performing the Qual Group 1 and 2 Unknown Analysis did not have any precipitate remaining after step 1–B. What steps should the student omit from the procedure? Explain your answer.
4. A student performing the Qual Group 1 and 2 Unknown Analysis did not have any precipitate remaining after step 2–B. What steps should the student omit from the procedure? Explain your answer.
5. For each of the unknown solutions below, determine what cations are present and which are absent.
a. After adding HCl in step 1–A to the colorless unknown solution, a white precipitate formed with a colorless decantate. After step 1–B, the precipitate completely dissolved to form a colorless solution. After step 1–C, a yellow precipitate formed. There was no apparent effect on the colorless solution after step 2–A. A white precipitate formed after adding aqueous ammonia in step 2–B. Finally a black solid appeared in step 2–C.
b. The blue unknown solution had no apparent changes in it after steps 1–A and 2–A. In step 2–B, a dark blue solution appeared after centrifuging, and a light blue precipitate was recovered. This precipitate was washed and became white. A black solid appeared in step 2–C. The dark blue solution lightened to a sky blue in step 2–D. Finally, a maroon precipitate formed after performing step 2–E.